I have had these things in hand about a dozen times and always wound up putting them back on the shelf. I see them in pet chains and notice a rather large difference in price between similar sized units. Petsmart has Topfin (way cheaper than the name brand they carry).
Sounds like this is absolutely right up my alley, but with something like 20 tanks running, I would like to make the onetime investment wisely and not purchase 20 units of one brand and wish I had done more research and gone with a different one.
Have you noticed major "differences" between brands? What brand(s) do you prefer? As a rule of thumb, what is the most "appropriate" magnet size for tank size?
There I assume is a "formula" to be most "appropriate" for a tank size. While the largest may always be the "quickest" it probably is "inappropriate" to use in a 5 gallon tank.