FreshwaterAfishianado
Fish Fanatic
You wouldn't put an Oscar is a 10 Gal. just because you have it hooked up to an fx6 would you? I don't think your filtration is at the heart of the stocking issue. Even though you may not be able to give the fish their exact habitat wouldn't it be better to at least give them enough room to display their natural behaviors? If the cardinals are very active fish could you create a constant flow across the tank using the intake and outflow of the filter to create a kind of stream cross-section? Or would the apistos object to the high flow? Just an idea