Need advice on ensuring tank saftey after fish death?

Sadly my Green Spotted Puffer ped away. I don't know why. He was in a brackish tank heater tank that was fully cycled when he entered. He did great eating snails and brine shrimp krill and the occasional treat of human grade shell on shrimp. But he stopped eating and I tried all I could to help him but he ped away. I had planted a few plant bulbs about an hour before he died and now they are growing fuzz so I'm thinking the tank is diseased somehow. I have kept the filter running and its been about 72 hours since he died. I would like to re stock the tank but I am scared to cause how do I know they will be safe. Also how long will the tank stay cycled with no fish should I add some amonia once in a while to feed the bacteria? My puffer showed no signs of anything except starvation. Please advise!