Fish keep dying!

I have a 20 gallon A 35 gallon and a 50 gallon. I have 0 issues with the 50 and 35. I have had nothing but issues with the 20 gallon. Over the past 2 years ive had over 15 fish guppies/mollies all die in this 20. Just thus last week I bought 6 new ones from petsmart and I was at 9 guppies. Now im down to 3. Water parameters are perfect. I use the expensive drop test kit from petsmart. There is also a mix of live and fake plants. I use a sponge filter and a charcole filter in every tank. Bi weekly water changes as well. I am at a loss of what could be going on. I had a full year where nothing died in this tank and now nothing stays alive. I've about had it!