Hi. I've been having trouble with tank.. I have a 15yr old pleco.that I've had for ever also a little spotted catfish plus three cichlids. Not really sure what kind. But I really have a hard time with water parameters. Everything is fine but the nitrates are way to high.. I've tried everything.. Water changes every week which I still do. To chemicals to new media.. In the tank I have aquarium fake rocks. Sand. And driftwood. I have two hob filters both are for 75 gal tanks. Which mines is 65 gal. I really only use seachem products besides there food.. I don't know why my water is completely off. And it has a slight yellow color to it.. Please help. I love my fish and none so far have died. But they don't even really swim around anymore