OK I have had a 55 gallon tank running for about 5 months now with 2 younger oscars, an eel, danio (don't ask he's just there) and a bristlenose. All very young and small fish. We decided to go with the new API filter nexx since we have never done canisters. Oscar poop wouldn't fit through the filter and I knew that was going to become a bigger problem when they got older. So I got a fluval 70 gallons. My water got really cloudy in the course of changing filters. So I hooked up the old hang on back and its still not going away after 3 days.

I am very ritual of doing water changes once a week. My water nitrates are at 5, where everything is 0. Salinity is at 1.002, temp at 77 right now. We have hard water, ph 7.8, hard water.

What am I missing???? Did I knock my biological cycle off with the filter change? please please help

I am not quite sure, I do know you should be able to filer 6 times the amount of gallons your tank can hold. So for a 55 gallon tank you should be filtering 330 gph. You could always keep both filters on the tank.

And yes when you switched your filters you basically killed all your BB and restarted the cycle.

As far as I know one of the best ways to switch filters is to run both and let BB build up in the new one and then take the old one off. You can always adjust the flow of water on the filter.

That is what I have read at least, if anyone else knows better I love to be informed

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You should have left your old filter on there for a couple of weeks while the new one got established. I am assuming you just removed it and hooked up the new one. I don't think you killed any of your beneficial bacteria, I think you just removed part of it and caused a mini-cycle. I would not feed for a few days and test for ammonia.

Cloudy water is usually due to a temporary bacteria bloom. It lasts for only a few days. Changing filter systems could cause it. Remember that when you have Hard Water you need to replace water lost from evaporation useing distilled water only. Other wise your tank turns into the Dead Sea through accumulated salts.

I didn't even think about that. I'm glad I hooked up my hang on again. That has all the old filter pads in it still. It makes perfect sense. Ammonia is perfect not even a change. Just the nitrates.