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Green Water & String Algae, Why do I Have it?

Updated: Sat, 11 Sep 2021

Green water & string algae seem to be the biggest bone of contention with pond lovers. Why do you have it and how do you get rid of it? Read on and find out.
Green Water & String Algae, Why do I Have it?

Every year, usually in June, we have a weekend long event called “Water Garden Weekend”. During that weekend, we have free seminars on all kinds of topics pertaining to ponds. The seminar that consistently has standing room only is the one about green water & string algae. It's a hot topic, and if you have either, or both, you're not alone.

The only way to understand how to get rid of it, is to understand why you have it. Algae grows, because there is an overabundance of nutrients in the water. It's really just that simple. Understanding why you have the imbalance sometimes isn't as simple. There are several reasons for your algae burden, here are the usual culprits.

First off, let me say this. Algae isn't bad! Green water isn't bad! It's when it goes out of control and is unsightly, that's when must do something about it. You want a nice short green fuzzy carpet of algae growing on your liner. If it's longer than an inch, then it's too much. Green water improves the colour of your fish, it makes it harder for predators to see your fish, it's full of zooplankton and is good! In a well filtered pond, with increased oxygenation, it's good! In an over stocked, under filtered pond, its bad! The other downside to green water is if your fish get a wound, it's not so easy to see.

1) Biggest reason for over production of algae is…..not enough filtration!
2) Too many fish
3) Over feeding
4) Not enough water plants
5) Not enough small water changes
6) Not cleaning the filter often enough
7) An accumulation of gunk on the pond bottom
8) Using the wrong plant fertilizer
9) No UV clarifier or Ionizer
10) PH/KH issues

Hands down, under filtering your pond is the biggest cause for green water & string algae. The best advice I can give you where filtration is concerned, don't believe what you read on the side of a box! If the box says it can do a 2000 gallon pond, it probably can't. Here's how you can tell if they are over stating their product. If it comes integrated with a UV, the UV needs to be 1 watt per gallon of water, minimum! So, 2000 gallons=20watt UV.

Also, there is no such thing as a self-cleaning filter. Anyone who tells you there is, is seriously misrepresenting a product. Run away from that person! There are filters that are easy to maintain & less maintenance, but no such thing as no maintenance. Everything needs maintaining!

So, how do you pick out a properly sized filtration system? First step, be honest with yourself! You need to be honest about your fish load. If you have 10 small Koi that are 4 inches long. Then that means you have 10 Koi that are 24 inches long. You aren't buying your filter for today; you are buying it for the grown size of your fish. Koi grow fast. Most likely, they will be 24 inches and larger in 4-5 years, more or less. We have 32 inch Koi that are 3 years old and a 33 year old Koi that is 24 inches.

If you have Goldfish, it's almost worse, because they reproduce like bunny rabbits! Those 6 Goldfish you started with in the spring can be over 500 by mid-summer. Small Goldfish only 3-4 inches long can reproduce. Where Goldfish differ from Koi, is that Goldfish can spawn (egg laying) non-stop from the time the ice if off the pond, right until the ice is back on the pond. Koi on the other hand, only spawn (egg laying) once a season. Each female Koi will only spawn once and not until they are 3-5 years old.

So, when you are choosing a filter, you need to keep multiplication in mind. For those of you who say you'll just give them away when they get big…ya okay! Most people fall in love with the fish and rarely give them away once they start following you around the pond and are hand feeding.

To assess how many gallons of water you need per fish. You add up their full grown size (12” for Goldfish & 24” for Koi) each Goldfish needs a minimum of 10 gallons per inch of fish and Koi need 15 gallons per inch of fish.

- A 12” Goldfish needs 10 US gallons of water per inch fish. Example 12X10=120 US gallons of water per fish.
- A 24” Koi needs 15 US gallons of water per inch fish. Example 24X15=360 US gallons of water per fish.

Again, these are minimums! The only way you can fudge these numbers, is by adding more filtration and more UV watts.

For Pete sake, do not believe that “expert” that tells you that you don't need filtration. That would be like an aquarium store telling you to load up your aquarium with fish and by the way, save your money, you don't need a filter. I bet you can imagine what that aquarium will look like in a week's time. Not to mention, most, if not all the fish will be dead. If you have a pond, it needs filtration, even ponds without fish.

There is no such thing as over filtering your pond. Like most things in life, they are only expensive if you have to re-do it. Buying an undersized pump/filter/UV will mean adding more, or buying all new. If you're not sure, please feel free to call us 1-800-KOI-TO-GO

Now that you better understand fish load versus filtration requirements, flow rates are also important but often misunderstood. If you have a 2000 US gallon pond, then you can have 5 Koi or 15 Goldfish (as long as you give away every single baby) 10 Goldfish would be better. You will want to turn your pond volume over a minimum of ½ or 1000 gallons, every hour.

Example; 2000 US gallon pond, turn over ½ volume every hour=1000US gallon pump. That would be if you were only circulating the pond. I prefer full turnover of the pond volume every hour. But to be fair, if you had a pressurized filter with integrated UV (best choice) then you'd want to buy either the Laguna 3200 with 25watt UV or the AquaScape UltraKlean 3500 with 28watt UV. Then you could safely put 2000 gallons of water through the filter which wouldn't be too fast for the UV. If your fish load was higher or you just wanted a bigger filter and UV combo, I'd suggest Pondmaster's PUV4000 with 40watt UV. That filter will be great on a larger pond of 3000 US gallons (box says 4000, but that is only true in dense shade with no fish)

UV's are extremely sensitive to flow rates. Too fast and the algae spores can't be destroyed by the UV. Too slow, and the algae cells multiply faster than the UV can keep up. So when people tell you that you have to go really slow through your UV, that is somewhat true, but also false  Please read the flow chart from your particular UV for guidance. Most UV's will honestly depict their product requirements.

However, there are some manufacturers that are quite misleading. I'm not going to mention any names (Oase) but they have a one size fits all mentality. Their pressurized filters routinely have 9 watt UV's in them. From a 500 gall pond filter, to a 4000 gallon pond filter. If you scroll up, you'll see where I told you the truth. One (1) watt per 100 US gallons. Where UV's are concerned, more is better. 9 watts will not work for a 4000 gallon pond filter.

So far we have touched on filtration and how much you realistically need. Plus we've discussed fish load. Common sense would tell you, that if you have too many fish, you are feeding too much. Seems obvious!

Some people really underestimate how much they feed their fish. Here's a pretty good rule of thumb. If you have string/hair/blanket algae in your pond, you're probably feeding too much.

Here's a good analogy for you, especially if you have kids.

If you put food out on the counter for your kids to choose from but the choices were ice cream or broccoli, which one would your kids choose? My kids would take the ice cream every time..lol. String algae is the broccoli, and the food pellets are the ice cream. If there's string algae in the pond, then the fish aren't that hungry. What we do, and it works like a charm, is we stop feeding for 3-4 days, even a week. We will resume feeding, half of what we used to. This ensures the fish clean up the pond and eat the best thing for them, their greens. Unless your pond if super clean, they will be just fine if you went away for a week.

On the list above, #5 talks about water changes. You should be backwashing your filters weekly (please see our article Summer Pond Maintenance 101) therefore you should be dumping out and replacing 10% of your volume a week. Please recycle the water as discussed in the article Summer Pond Maintenance 101. I could be a millionaire if I took a dollar every time someone thought evaporation was a water change. Well, not really..but you get the point  When water evaporates, it concentrates the impurities. When you fill it back up, it goes back to where it was, but a bit more concentrated. You need to physically remove the water to do a water change. You can exchange up to 20%-25% if you like, more just throws your pond balance off.
Here is a list of products that do help with green water & string algae. Please don't use all of them at once. We love the balls and Blast, as does anyone who's used them. We sell out constantly, so if you can buy what you need for the whole season at once, then you'll never have to wait.

1) AquaSpheres balls & Blast (this is what is used in every pond we have)
2) Pond Force sludge remover w/barley
3) Barley straw, pellet or liquid
4) D-Solv powder
5) D-Solv 9 liquid
6) Microbe-Lift PL
7) Pond Balance
8) Salt

- Not an additive, but part of your filtration, would be your UV and/or Ionizer

Water plants help with the excess organics and fish made fertilizer. Floating plant like water hyacinth and water lettuce are fantastic. They shade the pond, while consuming gross amounts of nutrients. Like all plants, they take out nutrients. The bad side to plants is they add to your organic levels in the pond. They make debris by decomposing. If you keep that cleaned out, then you'll be okay.

Obviously having debris on the pond bottom is not good, another reason we aren't advocates of rock bottom ponds. They are a maintenance nightmare and people that are into Koi, rarely have rock bottom ponds. People learn pretty quickly that all that gravel catches immense amounts of sludge and isn't good. Please read the article Summer Pond Maintenance 101 for more information.

PH is sometimes an issue with green water too. I'm not sure if green water comes because your PH is over 8.2, or if your PH is over 8.2 because of the green water. It's kind of like the chicken and the egg. It's interesting to note, that people without UV's, who have green water, tend to have a PH of 8.2 or higher. I rarely check our PH, for us KH is the important one. KH (carbonate hardness) is what buffers your water and holds your PH steady. A PH crash can and does kill massive amounts of fish overnight.

- PH should be between 7-8.6. Lower than 7 can be a precursor to a crash, and higher than 8.6 can make your water green. Your fish will get used to what your PH is, fussing with it will just stress out your fish. If it's much beyond these numbers, contact us and we'll help you solve it.
- KH should be over 80 but ideally between 120-250. Ours out of the tap is zero! That's beyond bad! We use Microbe-Lift KH Booster.
- Always test your pond water AND your tap water for comparison. Always test room temperature water, it's more accurate and use only tests kits that are less than 1 year old.

Some people are too impatient to find pond balance or want the pond to look perfect all the time. I just want to remind you, it's a pond and not a swimming pool. A healthy pond has algae!

Having said that, we have a pond, we call it our Savio pond. It's called that, because it has Savio equipment. It's also our pond that has the OGOPOGO in it. That pond has no string algae where the big Koi are, because the eat it. Where the problem lay's, is up in the extensive, super planted waterfall. It used to grow a 5 gallon bucket full of string algae every few days. I kid you not! We would use $600 worth of D-Solv powder in a season on it. It was time consuming and drove us nuts! So, 3 years ago we installed an AquaScape second generation Ionizer on it. The pond is 6600 US gallons, has 10-14 Koi in it and was string algae free in 5 days. Not one speck of algae all season long, it was wonderful!

I think it would be highly irresponsible to not scare you a little about Ionizers. They work by a copper probe releasing low levels of copper into the water. Algae are also super sensitive to copper. The goal is to find the happy spot for your pond.

- Koi are EXTREMELY sensitive to copper. Much more so than Goldfish. If you buy an Ionizer, I guarantee that you won't have algae in a week or less on average. If you do not follow OUR instructions to the letter, you may very well kill your fish!
- If you buy the Ionizer and set the LED pad to the highest level, then go on holiday for 2 weeks, you may have a problem!
- If you buy the Ionizer and set it to 10 and don't monitor your copper levels or notice when your fish aren't happy. You will have a problem!
- If you follow our tried and true instructions, you will be algae free, with happy fish!
Please call us for instruction 250-765-5641 or 1800-KOI-TO-GO.

Ionizers are a fantastic option for people at their wits end. The have saved many a pond from being filled in. They can make the enjoyment of your pond, like nothing you've ever experienced before. They are relatively expensive at nearly $400 but worth every penny! You will be sitting by the pond having your coffee or cocktail, rather than pulling string algae. You will love your pond again 

In summary, green water and string algae issues are usually because you are doing something wrong. Whether it be skimping on filtration or UV. Maybe you are like everyone else (us included) and you have too many fish. You love your fish too much, so feed them constantly. You are tardy on your maintenance, and water changes are too little and too few in between. Maybe you are getting upset and dumping the pond and cleaning it out, only to find it green 3 days later (that's a big no no!) Maybe you aren't cleaning your filters right.

It could be lots of things, the biggest thing is patience! When you are ready, come by and talk to us about your issues, we are here to help. We promise to not make too much fun of you..lol. Honestly, we've all been where you are and we're here to help! Just ask

Happy Ponding