what is your ph? don't do any adjusting without knowing that parameter.
all my snails i raised in water with a ph of 7.8 without any issue at all, and my water is hard enough to leave water spots on dishes that air dry. fish nerds here say the water we have is perfect for african cichlids due to it's hardness. you don't want the ph too low because that can cause shell issues (soft water isn't good for snails).
i've also used ferts in my tanks with apple snails, and dosed more than i should have (oops! lol). no issues at all, no sliming. since you've done all these water changes, and removed the terra cotta, i don't see much for residual ferts bugging the snail at this point.
edited to add: the ferts i've used have been Flourish and API Leafzone (i think? it's been a while).