What's funny is my friend, who has a 20g with a ram, gourami, a school of tetras, a school of corys, and a BNP, hasn't done a water change in almost 2 months. He just tops his tank off. I'm thinking he has no issues because he has about a dozen stems of Anacharis, each about 16-20 inches in length.
He needs to do a nitrate test -- not with the dip sticks, which are next to worthless --but the test tube variety. API makes a good one. I bet it will be glowing red like neon. There is no substitute for big water changes. A redwood couldn't eliminate the nitrates from my tank.
And Firemouths grow slow. I had no idea mine would look as cool as he does at 4 years when he was a pup.