I have to add that our tank was just fine for a few months as well. Then, one fateful day, we bought 3 otos. A week later, a few of our fish had white spots. We started treatment immediately, but despite our best efforts, a month later 14 of our 18 fish had died. So, now we have a quarantine tank. It takes a lot of patience to be an aquarist. We quarantine our new fish from 3 to 4 weeks. So far, so good.
Recently, we had one more fish get parasites, and we removed it immediately from the display tank and put it in the quarantine tank, which luckily was empty at the time. So far none of the fish in the display tank have got the parasites, and the sick fish is all better. His treatment is almost over, and we'll wait a week or two before we put him back in the display tank.
It cost about $30 for the 10 gallon tank, heater (already had an old HOB filter that does just fine for the Q tank) and thermometer. We don't light the Q tank, and just put some hornwort in it to help with filtration. I highly recommend the Q tank. After 6 months of having no Q tank, we basically had to start all over just because of that one incident!