Frustrated

[Broncovin]

I have had my tank up and running just over 6 months now.
Breakdown - 30 gallon
2-aqua clear 50's running
Bubbles and underwater fan for movement
4-Molly's
4-tetra
2- catfish
1-algae eater (placko)

Cycles went very well but I have always had an issue with ammonia
I do constant water changes 25-50-75 and 100%
Still ammonia remains
In the tank is just gravel 2 small aquarium safe rock formations and 4 plastic plants

Now I find the fish eat very little or not at all so I have tried changing food with no luck

Don't know what to do. I have over $700 invested
And all I want is a happy healthy tank

 

[Mumma.of.two]

What do you use to test?
What is the ammonia reading?
What's your tap waters ammonia reading?
Do you change filter media regularly?