Yes, you really should have a basic set of tests, namely ammonia, nitrite, nitrate and pH. The API Master Combo has these four, so that would be less expensive than individual test kits. Th nitrate and pH should be tested periodically, regularly before the weekly water change for a few weeks, then sporadically as spot checks, until you have a good idea of the tank's conditions. The goal is that nitrate will never rise above the lowest it can be in your setup, and the pH remains the same within a decimal point or two. Ammonia and nitrite are good tests to run when you see a problem as here.
Keep an eye on the Red Tail Shark. This species is well know for not tolerating loaches, especially those with vertical stripe patterns (no idea why, it just is), and even if physical contact is not seen, the RTS may still be harassing the clown loaches and that means stress, and stress is the cause of ich. The protozoan has to be present, but it is stress that prevents fish from dealing with it and with loaches it can be deadly. Now that I've seen the other species, I would increase the heat a degree or two higher than 30C/86F, as these species can manage that for two weeks. Ensure good surface disturbance.
