For help in choosing the correct treatment for fish, visit this website.
http://www.waterlife.co.uk/waterlife/disease.htm
***Note: I'm not sure if waterlife supplies stores out of the uk ***
At my work, we have 150+ tanks, which are all filtered by several systems. There is a very large filter for every 5 tanks, plus heating. We also have individualy filtered tanks, that use under gravel filtering.