ywng, im using just tap water now too...find it a hassle to buy RO water from the supermarket for the tank.....need quite an amount.
Mineral water is definately a nono though, unless you are looking to grow algae and nothing else in your tank...
Cards do work..and LFS sells those tank cleaners which i find pretty effective as well, at least u dun have to get your hands wet...~ Impt thing is do occassional cleaning. Let the algae grow over time and u'll hv a hard time cleaning it...
Erm, they sometimes help clean parasites and dead tissue from your fishes....clear up left over food. Have to say that they don't help with the parasites v often...a cleaner warrse also known as the doctor fish does it better but they are known to have a high mortality rate in home aquariums. My cleaner shrimp is just for show....hehe. Pretty interesting to have shrimps in the tank...
Erm, heard boxfish will release some toxins when it gets stressed or dies. Maybe that why your other fishes died? Anyway, maybe you can do a major water change and monitor. Gd luck....
Hrmz, heard some reefers avocate leaving the tank fishless for around 8 weeks. This will ensure that your tank is ich free. Not sure if changing water will help.