I'll start out by stating that I'm fairly new to the whole fish-keeping scene, so decided to start out with what are supposedly one of the 'easiest' fish to keep, Fancy guppies. For reasons that I can't work out however, they have been dying off at a rate of approximately 1 per week over the last month or so. I do not see any signs of infection (no white-spot, etc) and while one had a bit of a tail attack (bite marks), I don't believe that is the cause (the one i scooped out this morning has no such marks - i actually believe he may have been the 'bully' in the tank doing the nipping).
The tank is a 10g 'wall mounted' tank, planted with some lace fern, mondo-grass and anubias nana (on wood) in eco-complete substrate. It is the home of two catfish (1 Bristlenose and 1 Cuckoo), a handfull of annoying snails that came with one of the plants, and what is now 3 male fancy guppies.
I've been doing weekly water tests which generally show pretty stable readings as below:
GH: 15dGH (this does seem to fluctuate a bit, from around 13dGH to 19dGH - the biggest jump I ever had was 3dGH in a week)
Temperature: 27ºC (81ºF)
I do twice-weekly water changes of about 25%, generally including water conditioner, API StressCoat, Flourish and Flourish Excel. Whenever I add new fish, I add the recommended dose of PRIME.
Any ideas on what could be causing these 'hardy' fish to keep dying off?