Well we asked for more Info on you setup, and never got it. I will gladly offer advise once I learn more about it.
OP asked if he should add another filter. He has a 40 gallon wet/dry that's filled with bio balls. If he's having an ammonia problem, he either has something in the tank causing it or a lack of bio media to break the ammonia down. I believe that that would solve his problem. The tank has been running for quite sometime now but his fish are getting bigger and he might be overfeeding.
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