90* is too warm for the fish, about 80* is the max. You do have to keep an eye on heaters, the built in thermometers are not always accurate.
As for the nitrite/nitrate, I'm not really sure. If your tank is cycled, you shouldn't have nitrite at all. Nitrate can weaken fish, which would make them more susceptible to disease. If you want to keep nitrate down, (which I suggest you do) you can try doing daily water changes of about 2 or 3 gallons. If it is convenient, you could do this with a vacuum to remove detritus, which would help even more. If you really want to put a lot of effort into nitrate removal, you could remove most of the gravel, so you can vacuum up the detritus more effectively. However, before you go too heavy on nitrate removal, do a water test and see what the level is. If you don't have a kit, a lfs or big box pet store should be able to a test for free to cheap.
They may or may not be eating the cucumber, the only way to really tell is to see them eating it.
As for the rash, no clue. You may have sensitive skin, or there may be something in the water that is irritating it. Keep in mind, even if that is the case, it may not be bad for the fish. I honestly just can't tell.
Good luck,
Brian