Sorry to hear about your loss; this is the kind of thing that turns people off to Tropheus
, understandably. Metro is more of a prophylactic than a cure: treating a colony may save fishes in the early stages of infection, but it rarely cures fishes with overt symptomology.
To your questions-
(1) 'Bloat' is not caused by an exogenous pathogen in the same way that diseases like 'ich' are; it is caused by organisms that are normally present, but produce opportunistic infections in fishes whose immune systems are compromised by chronic 'stress' and other ill-defined variables in the environment. In Tropheus
, the most common causes of 'bloat' are inappropriate diet and/or water chemistry, but other factors- shipping stress, not enough tank space, inappropriate tankmates, etc.
- can also be contributing factors. Which leads to your second point-
(2) Cleaning up the fishes' environment, including the canister filters, is useful, but as described above, the organisms that cause 'bloat' will always be present. You need to assess why your Tropheus
colony got 'bloat' in the first place, and correct that. The fact that the entire colony was affected strongly suggests that there was a systematic cause (perhaps diet?). You need to get a handle on this, as Mbuna are also susceptible to 'bloat,' just less so.
Don't be discouraged; this happens to everybody at one time or another. Good luck,