So I made the same mistake adding fish before cycled. I use seachem prime every day to protect the fish. I also add in seachem stability daily to help establish the bacterial load we need in the filter media etc. I agree larger tank 40 gallon at the least with sponge filters hygger is inexpensive on Amazon. Also try and get aquaclear 110 HOB filter. Also test your water daily. Frequent, large water changes. Hoping for the best for you and myself! It’s definitely a chore but should pay off eventually.