Where these the fish you cycled the tank with? Did you add all the fish at once or slowly one by one?
If you added all the fish at once then your tank was overloaded during the cycling phase possibly causing stress and long term effects on the fish used for cycling. That by itself would be a reason for you loosing some of them now.
If your tank is finished cycling which I would assume considering the time it has been up and running, your ammonia and nitrite should be 0, but you should have some nitrates.
For the stocking level of your 20 gal tank I would recommend you do a gravel vac and weekly water change of 25% but do not clean your filter or pad every time. That is were the majority of the biobugs live that keep your tank healthy. By cleaning the filter and pad you destroyed them and their home and that may well have caused a problem for your tank. I never clean the filter housing and piping at the same time I change the pad and I only do that every 6 months or so, when the pad falls apart.
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