Chemical Test Kit
A test kit is an absolute must-have item for me. If I see a problem that's not outwardly obvious with my fish, I can test temp, ph, ammonia, nitrite and nitrate to get some clues. I use the FasTest (formerly SeaTest) kit by Aquarium Systems.
Some of their test chemicals are interchangeable with both fresh and saltwater. I emailed them that question and here's the direct quote from Aquarium Systems:
Use in fresh water:
Ammonia - you can use it in fresh, but the reading may not be exact, it will produce yellow color with zero ammonia and will produce green colors with ammonia present so it will tell you what you need to know
Nitrite - will work in fresh water
Nitrate - if using in fresh water divide the reading in half
pH - will not work in fresh
You can use the FasTesT reagents with your old SeaTesT vials/chips.
UV Sterilizer
The UV works very well for me. Be aware it can only kill parasites & bacteria in the free-floating stage, so you shouldn't skip the usual precautions. After going through a serious bout with ich in my main tank, I'll never have another tank without one!
The UV only kills what passes thru the tube and is exposed to the light, so the attached good bacteria in the tank isn't harmed. It also kills free-swimming parasites without stressing my fish. I run it constantly.
A couple of cautions:
Wait until the tank has finished cycling before hooking up your UV!
Don't run a UV when medicating; the light can change the composition.
To maintain effectiveness, replace the bulb every 8 months.