Hey all. I have had tropical freshwater tanks for over 10 years now and usually never had much issue, until now. I got a new 30 gallon tank from my sister who was done having a tank. I set it up, let it cycle, then moved my fish over. They all seemed happy and well. This was about 6 weeks ago now. In my tank were 4 male guppies, 4 neon tetras, 1 emerald green cory cat, 3 peppered cory cats, and 2 oto cats. Now 3 days ago my tank went green overnight. Pure green water. My fault really, I left the big window open and shining on it and didnt do the weekly water changes the past couple weeks like I should have. Figured normal algae bloom. So I put in my algae killing drops and did a small water change. Waited 2 days, like it says, and did that again. Almost immediately 2 of my guppies and 2 catfish started acting like they had bloat. So I did a massive water change (about 75%) because my main worry was the algae killing drops were harming my fish. I got up this morning to all 4 of those fish dead, my other guppy isnt doing well and 2 of my neons have lost their neon. I use testing strips for water testing and show zero nitrates and nitrites and 7.0 ph and both hardness are within range. I do not have an ammonia tester so I dont know those numbers. Could my algae killer have done in my fish that quick or do i need to look into something else? I weekly test my water but changes have gotten hard with theis covid stuff and me working lots in a hospital. Any suggestions on what else to look into or buy to test my water or anything is helpful. I hate seeing my fish suffer.