So when I first started this new hobby I was extremely unaware how overstocked my tank was (4 tiger barbs, 2 sterbai corydoras, and a pleco). I only had a 10 gallon tank and was constantly having ammonia problems. I scavenged all thrift stores for a new tank and luckily found a 20 gallon for a good price. When I started this 20 gallon I used my first 5 to 15 gallon aqua tech filter and bought a new 20 to 40 gallon aqua tech filter and have been using both. After 3 days it was as if my fish started to get sick and die a fish died every day for four days. One fish would swim to the top and just hang out while all the other fish would continue like it's normal. It has now been one week since the death of four of my fish and now I have only 2 tiger barbs and 1 pleco in my 20 gallon. I do not plan on getting anymore fish as of right now. And would only like to make sure this does not happen again. I want to make sure my fish are happy and healthy any tips, recommendations to prevent any illnesses and water problems would be awesome I appreciate all replies.