I understand perfectly.
It may take a while to get on top of it. As there are so many snails, if they do start to die off because of lack of food, their bodies will sink to the substrate and start to rot quite quickly. This in turn will produce more waste, making it very important to keep on top of substrate cleaning, water changes and water testing.
As ColinB has said, there is a lot of goodness in the substrate, so too much cleaning has a detrimental effect. Looking to remove obvious waste, such as discarded food, dead plants and fish/snail poo.
Water testing is absolutely vital of course.
In your previous report you gave:-
pH: 6.5 - 7
Ammonia: 0
NitrIte: (lowest colour) <- should be 0ppm
What about NitrAte? Although NitrAte isn't harmful in smaller quantities, it can be a problem if it gets too high.
Also, what type of water test kit do you use, liquid or strips? (Liquids are more accurate)
If you test the water again, be sure to do each of pH, Ammonia, NitrIte and NirAte and post it here as you go. It will help us all get an idea of what's happening. Test your normal tap water too to get a gauge on any differences between tap and tank. Keep a glass of tap water at room temperature for 24 hours, and test the pH again after that. This allows the gases to dissipate and give you a better pH reading. This doesn't change for me, but others it does, just depends on your own water supply.
If your pH gets too low it can affect the Nitrogen cycle completely, though I have only had that problem when setting up my 64ltr tank. At a pH of 6.5 it is getting quite low. If your tap water is higher, then at least we know how to help the balance a bit.
Again, as per ColinB's suggestion, no meds until water is fully tested.
Don't mix meds, use only one type at a time when trying to treat different things.
Make sure you remove the Carbon filter/bag from your filter before dosing meds, and leave it out.
After the run of treatment, put a Carbon filter/bag back in for a few days to help remove old meds, then take it out and throw it away.
Then start your next medication.
Water changes should be done after doing any cleaning to allow more contamination to be removed, dose medication after a water change. Water testing should of course be done first.
Phew, that's a lot to take in and I think I've got the basics down. Others will chime in if there's any finer points that need talking about.
Anyway, first thing is to get the water tested fully so we can see what's going on.