vinylman
Essentially Humanoid
Now on day 3 of using API's Melafix antibacterial treatment in the tank, and keeping a watchful eye on the surviving inhabitants.
Thanks for all the advice, I'll start doing more frequent water changes, I just didn't think there was anypoint as all the levels were pretty much exactly the same after a change as they are before, (ph, Ammonia, nitrIte and nitrAte). In fact I only settled on monthly water changes as I use a 'gravel syphon' to take the water out and doing it monthly keeps the gravel nice and clean without taking so much out that the plants suffer.
Will let you know how it pans out...
As you have a small tank, more regular water changes are probably the best preventative measure you can take against illness/disease. In a small amount of water, things can go very wrong very quickly. Weekly is definitely the way to go. That said, I personally wouldn't touch Guppies with a bargepole these days, unless you know someone who breeds them; they'll be so much better than any you get in the shops. They should also be kept in a ratio of (at least) two females to every 1 male - otherwise they'll be pestered to death by the males; that stress would also leave them susceptible to disease.