Backstory - If you don't care skip to the bottom :)
I'm getting better at this hobby, but I'm still new. I keep up with my 55 gallon Mbuna tank. Every week I do around a 30% water change at the end of the week before the water change Nitrates are at around 20. I had a huge 8yr old Female Kenyi cichlid, 7 yr old Red tailed shark, 20yr old common Pleco that my mom bought in 1990, 3 yellow labs, 3 Yellow tailed acei, and 2 bristlenose pleco. One yellow lab was holding and my BN plecos had a fry. For the first time with this tank and the second time since I entered to hobby, I purchased fish that ruined my setup. three weeks ago I bought 1 more yellow lab and 1 more yellow tailed acei. Two weeks later, the Kenyi stopped eating, bloated and died (rather quickly). Then last week all on my fish started flashing, color looked like crap and some were swimming strangly , some of them were not using their fins. I checked and I had 0 ammonia, 0 nitrite, 10 nitrate, ph 8.1, temp 79. I did another 50% water change and I treated with Coppersafe thinking they had a parasite. After a few days no improvement. I added salt to the tank (I never put salt in the tank). The next day the remaining of my cichlids looked the same and were still flashing. More concerning my Red tailed shark was covered in clear slime and her eyes were gray, she died that night. I added Melafix to the tank. This morning the yellow labs that wern't using their fins were using them again, the aceis still look bad and my pleco is swimming way more than normal and it looks like he will sadly die after being in the family for so long. I'm plan to continue to treat with the Melafix and do a few extra water changes and hope that this disease runs its coarse.

SO I AM CONVINCED, having a QT is a must!. I am buying a 10gallon because it will fit inside my 55gallon stand. I am going to run 2 tetra 10 gallon filters on it and cycle it with ammonia and use ammonia to keep it cycled. I'm looking for advise on what I should treat my QT tank with when I buy fish. They will be treated no matter what and they will stay in that tank for atleast 4 weeks.