Ack! Whats wrong with my Rainbow Shark?
a 55 gallon tank is a minimum for a rainbow shark they grow 6 inches plus you have the pleco who are huge waste producers My honest opinion is your water Quality is bad i know you said its been tested but it cant be perfect if it was there is no way you would have popeye My guess is ammonia and nitrite in your tank and the guppies died because your water conditions are bad at the shop they would of been perfect and you added the fish and they could not tolerate the conditions and have died.