That could be a possibility. I know they say to never spray anything in the same room at the tank itself. Another thing I have heard is when people clean the aquarium glass, I use nothing but a damp cloth and water. I then wipe it dry and it works great. Or if you have to use a glass cleaner spray it on the cloth first in another room and carefully clean the glass making sure you don't get any on the lid or any area that the moisture hits it as it can then leach into the tank.
Your two clowns were they they ocellaris type (nemo)? We had a huge problem here in Canada where I live with these particular type of clowns, they were dying left right and center. I don't know what the problem was with them but they sure were not hardy at all.
When I got my two they were fine and I have had them for 2 years until recently I traded them for a Maroon Clown. This is one of my absolute favorite, they are so velvety looking and just all around beautiful in my opinion. My trade was also due to my dwarf lion fish sizing up my little male.
Maybe you want to give a different clown when it is time a try. There are lots out there to choose from. You have a nice size tank to work with so if you go slow and add a few fish at a time and see what happens.
Another thing that popped into my head is the filters. When I had my 10 gal nano going I bought a fitler/powerhead thing called a stingray, it was the shape of a stingray and this filter was positioned inside the tank. Everything was fine and oneday I noticed the fish not looking so good. I then seen an oil slick on the water surface of the tank. Low and behold it was coming from that dam filter. In the garbage it went, huge water changes and alot of praying I save everything. I am a real stickler now about placing any type of filter into the tank. I have never heard anyone complain about the powerheads so I guess they are okay. I don't use one as I have two Aqua Clear Filters on my tank and the movement is great, I would blow everything out of the tank I think with more movement but in a tank of your size you would need one or two. This is just another thought about checking the equipment.
As for corals, I am no expert at all. I only have assorted mushrooms. If you wanted to try something before the fish mushrooms are cheap enough and get one cluster and give it a try. You say you have a clean up crew in the tank, have you lost any? They sure would feel the affects if there were anything wrong with the tank.
As for the ich, if the fish are badly infected with it I think it can certainly kill them eventually. Anything like this is stressful on fish. But I think your clowns probably had more than just ich bothering them. If you have a different store in your area you might want to try getting a fish or two from them.
I don't often buy fish from a store I order them in and pick them up right when they come in. That way they don't go into the LFS tank which more than not has ich. I have never had a problem with doing it this way and much prefer it too.
Good luck, I hope I have helped in some way. I am a learner too being in the hobby for only 2 1/2 years now and have learned with ups and downs too.
Below is a picture of my new maroon clown, he is missing his top fin due to the idiot that caught him. But it will grow back in no time.