Is My Pond Filter Too Small?

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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Todays Q & A Of The Day comes from Susan who writes, “HELP! I think my filter is to small for my pond. I have a Laguna 2100 bio filter. My pond is in full sun it’s 23ft.long 20ft.wide and 3ft.deep. My pond is 3yrs. old and my water is still green and my ph is to high. I have used all kinds of chem.and solar salt. Nothing has changed. I can’t believe my koi are still happy and alive! Can you help?”

Answer: Thank you Susan for your question. Fish can certainly handle green water and if the pH is around 9.0 or below you’re probably ok there as well.

For green water the sun can play a major role so I like to add surface plants if I can…assuming the koi won’t eat them. You’ll want to get a gallon estimate of your pond, that’s the first step in determining the right filter size.

Based on the dimensions you sent along I calculate the pond to be around 10,300 gallons. I’m suspecting the 2100 laguna won’t handle that but check the manufacturer info on that to be sure.

The other thing you need to consider on this is the fish loading. The more fish you have and the larger they are the more the filter will have to work. So let’s say someone wants a fair number of large fish in a pond of your size…I would then shoot for something to handle maybe 12,000 to 15,000 gallons just as an example…overdoing it a bit is not a problem, but undersizing a filter for the demand is.

For fish health the main things to monitor are ammonia levels, nitrites and nitrates…the first two are toxic to fish and will come about if a bio filter is overwhelmed. The latter is very supportive of plant growth including algae.

I would definitely do a bit more research on the filter and see if you need to increase that, add plants if you can which are natural filters in a way, and monitor water quality issues as suggested to make sure all is well for the fish.

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