Join Date: Aug 2010
Location: Beaverton, OR
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Re: Very High Nitrates and Nitrites but normal Ammonia
You can get away with it for a little while with double filter capacity, but it's not a long term fix. The whole reason overstocking works at pet stores is because the fish aren't staying there long term. Overstocking in moderation works long term when you have a double-sized filter and weekly 50% water changes, but you've gone way past that. Just to get an idea I plugged your setup into the aqadvisor calculator, and you're at the 224% stocking level, only 94% filtration for those species, and would need to do 60% water changes weekly to keep things healthy. That's EXTREMELY overstocked, not just a little.
Edit: Another thing to bear in mind... The type of tank you have is very poorly suited for overstocking. Overstocked tanks do better the more surface area they have for gas exchange, but in a column you have even less surface area than a standard 15 gallon. It's one big recipe for failure.