the only safe value for ammonia or ammonium is 0 regardless of PH you'll need to do more water changes to get it down and keep it down.

if you seriously want to keep your pleco, get onto ebay and keep an eye out for a 4x2x2 foot tank, thats the size you'll need.

as for your discus he/she is in VERY bad shape, if it were my fish i'd be reaching for a bucket and some clove oil.

if you really want to give it the best shot of survival, get it into a bare bottom tank (or big bucket) with a heater and airstone, do daily or 2x daily water changes or around 95% and feed live worms, i know this will give the fish all kinds of diseases, but if you try and treat aggresivly in that state it will die.

live worms are very high in fat and will put bulk on the fish fast, you'll see improvement within 2 weeks. once the fish has put on some weight worm with prazi and levimenisole, do at leats 4 treatments of each then if need be have some metro on standby to treat for protoza infections