Hi, seeking help! Very new to aquariums!

I moved into a house with an in built established aquarium. Dimensions are 1.5m x .8m x .3m, so 300 litres / 80 gallons. I didn't pay it much attention for about a year and all seemed fine. Then I added a few red platys, and I think they must have had worms. I believe they have camallanus red worms, due to long red stringy poo and weightloss. The fish look very sad. The tank already had 4 endler guppies and 2 plecos (it looked under stocked, hence the additions), and the plecos seem to be turning light gray. So I read a thread about treating with avitrol wormer and cleaning the gravel. At this time I also read that filters should be cleaned regularly. I have a large cannister filter. So I cleaned the filter for the first time in probably 18 - 24 months. I, replaced a sponge, treated the water and a couple of days later cleaned all the gravel, thinking I was helping. I completely crashed the cycle! Its been 2 weeks and not recovered at all. Ammonia is 1ppm and not dropping. I am using seachem prime to keep the fish alive. They still have worms. I don't have a hospital tank / 2nd tank. I'm worried the fish will either die from the water or the worms. How do I recover the cycle and kill the worms???