Please keep in mind the reason your pH is dropping - your tank load of 6 gouramis and a clown loach are producing enough waste to acidify the water rapidly. Acids cause pH to drop, and this will happen more readily without the buffering agents in the water (KH in this case), as discussed above. This is a valid issue, but you are going to be constantly battling your pH and KH, "chasing the dragon," so to speak, with a heavy fish load like you have. If you have less acidifying processes going on in the tank the pH will be more stable, even without more buffers.
Gouramis can be territorial and aggressive, even, so having 6 in a 15gal can certainly prompt an outbreak of ich, though the stress of adjusting water params will do it also, as you suspect. You may also be aware that the clown loach has the potential to get extremely large, and incidentally they are very prone to ich.
My approach would be to go down to two gouramis and the loach (for now, as he is small) and consider a small school of tetras to fill in some space. That fish load would be much easier to manage in terms of keeping water params under control, and it would be more comfortable for the gouramis.
If you feel the fish you currently have are living harmoniously, then I'd switch to a religious water change schedule of at least 50% per week, or split that up in twice-weekly PWCs. A change in PWC schedule needs to be done gradually, as the fish are acclimated to the current params, especially what I would guess is high nitrate.
I certainly don't mean to preach or tell you how you should keep your tank, but I just want to point out the reason behind the issue that you are trying to correct. Best of luck!
