In And Out Of Cycle ?

My 20 gallon was set up about five months ago, maybe six . It took forever to cycle. I did a fish in cycle. I ended up buying a “ used” sponge filter from Angels Plus to supplement my aqeon filter and that seemed to help. I finally got to 0 Ammonia 0 nitrites and 0 nitrates. I did regular water changes faithfully. Now I am back to seeing ammonia again.
I feed them once a day and vacuum the substrate a couple times a week.
I am starting to think that my tank is overstocked and that’s why it won’t stay cycled. There is no way I can get rid of some of the fish. They are all doing well and I haven’t lost one in months now. I wish I knew more when I started the tank but it is what it is. I have two angelfish which most people tell me are too big for a 20 gallon tank . They were tiny when I got them and have tripled their size and seem to be thriving. There are two mollies, 2 platys, 2 small cat fish, 2 ottos, and 3 guppies.
The guppies came by surprise. I bought one guppie and a week latter she had babies in my tank. The mother died and the three are what are left of the babies. Seem to be doing well.
Will my tank ever stay cycled?

 

It doesn't sound like it ever was cycled. If it was you should have had nitrates.
What is the GPH of your filter?
Just checking you use dechlorinator right?
What is the pH?

 

It doesn't sound like it ever was cycled. If it was you should have had nitrates.
What is the GPH of your filter?
Just checking you use dechlorinator right?
What is the pH?

I did have a nitrate reading at one point. Yes , I use prime every time I do a water change ( in the new water before I add it to the tank. PH is 6.4

 

You may be under filtered now that the fish are bigger. What filters are you using and the gallons per hour (gph)?

 

I did have a nitrate reading at one point. Yes , I use prime every time I do a water change ( in the new water before I add it to the tank. PH is 6.4

A ph of 6.4 is pretty low for the bacteria. It can grow but doesn't do so well. The ph may well have hindered the cycling.

 

With a pH of 6.4 it can be difficult to cycle a tank. At a pH below 7.0, ammonia starts turning into ammonium and by the time your pH gets down to 6.0, all ammonia has turned into ammonium. The good thing about ammonium is that it is far less toxic than ammonia (some claim ammonium is non-toxic). The bad thing is that ammonium is a terrible food source for the ammonia converting bacteria and you will have a difficult time developing a bacteria colony.

 

You might need to modify your filter because it does sound like you've overstocked your tank.

Some good mods if the fit in your filter are fluval biomax which are ceramic and give your bacteria more room to live on.

If you don't have room in your current filter to add some more media, you might need to buy a larger filter with space for it.

Also, do you have a planted tank? Plants help with the health but the most important part us your cycle.

Considering what jdhef said about the ammonium and ph, its possible you had a cycled tank but your bacteria died off when you ammonia turned to ammonium. So, you might get some Tetra safestart plus to get your bacteria levels back up before your ammonia goes too high. Before using tss, though, read the below link. It's from one of the forum mods who wrote to tetra for more specific info on tss and it explains exactly how and when to use it for good results.
Q & A With Tetra about Tetra SafeStart

 

I love TSS but if you pH is too much below 7.0, TSS may fail, since the bacteria will not feed on the ammonium and will starve off.

 

You may be under filtered now that the fish are bigger. What filters are you using and the gallons per hour (gph)?

I’m using an aqueon quiet flow for up to a 30 gallon tank along with a sponge filter . Not sure what the gph is on that model .

You may be under filtered now that the fish are bigger. What filters are you using and the gallons per hour (gph)?

I believe its 200 gph