20 gallon of death...

So, I lost a few males then female Bettas in this tank. I cleared them all out, cleaned the tank & later over two days, moved in Platy. 30 total originally, but now down to 5-7(3 days later)! mg:I acclimated them, let them calm after catching them, but still, they died like flies! Tank is showing 0's for Ammo, Nitrite, Nitrate, stable heat. So wtf!?!
I wanted this tank to be a breeding tank(now it will be a plant tank), but I can't do that if this tank really is a Tank of Death!
Noticed two Platy have clear, white, long poo. They really haven't eaten in a few days, not sure if that's the reason.
Anyway, anything I can do short of tossing everything and w/e I want to keep, soaking it in Chlorine??

No, it's a newish tank.
Yea, I did it over 2 days, but it was a little too much. Levels stayed the same, I did 5gal w/c 2x a day usually too.

Weird update. The 8 I think, Platy were swimming around this morning before lights on time! I was amazed. They all looked dead last night. I haven't done a w/c yesterday, so I will leave them be as much as possible to let them keep getting adjusted.
Maybe the catching was way too much stress and then the new tank didn't have enough hiding spots...+ bad luck. >.<

Parasites, what kind and how do I kill them?