However Melafix tends to drop pH. While most bacteria will die by the time a tank plunges to a pH of 5, the guppies will already be dead. :(

Syhrus does have a point about stress from a dominant guppy. That can indeed retard healing. Older fish tend to not heal well anyway and he again may be spot-on there too.

Non-dominant fish get sick easier. Indeed, in case of an epidemic, one can watch the fry get sick and die first and then watch the adults lowest on the pecking order get ill. When they are gone, the more dominant fish are in trouble. You may have to move that male. "Stress kills" has been a common theme for one revered aquarium magazine columnist for decades.

You mention that you have him in a small tank, though you don't say how small. I just mentioned to G ma that small tanks are very difficult to keep safe for our fishes. And male guppies are probably more vulnerable to waste materials in the water than a number of other aquarium fish.

A survey on GL sometime back asked what the best beginner guppy tank would be. A majority of votes went for a 10-gallon tank, because of prices for the tanks and equipment. They are also easier to clean and move than larger tanks. However a good case could be made for 20-gallon tanks. They are much more stable.

I know that people will complain about space and money issues. I do use small tanks some, but have to do more to keep them viable. I'm actually getting rid of smaller tanks and replacing them with larger ones, as space and finances permit.

By the same token, we don't have a large dog, which needs lots of room and a field to exercise in. I walk a 22-pound monster about 1.5 miles/day. Ok, actually we amble 1.5 miles. ;) If I jogged, it might be called jiggling.  We must try to scale our pets to what we can provide for them.

A couple of quick takes:

'I change about 50% of the water every week.'

Assuming that is properly treated and seasoned water, there is very good. However, until your male heals, try doing that three times a week. And make sure that no food goes uneaten. Maybe feed the same amount of food in more (smaller) feedings. I know with many busy people's schedules, that is difficult.

Better water quality will also sometimes moderate aggression. It can even effect fertility and size and the vitality of fry.

I once did 40% water changes on a rainbowfish tank for six or consecutive seven days (after being out of town for a family funeral). They went from dying to egg-laying. A fish producing viable eggs   is a healthy fish. If I waited to find out what was afflicting them, it would have been too late. But I might have been able to authoritatively have declared what wiped the entire colony. ;)

"PH always seems a bit higher and I try to bring it down."

What is the pH? Yes a really high pH makes ammonia even more dangerous than it is otherwise, but guppies are more or less hard water fish, 7.4 is almost low because the bacterial actions of the nitrogen cycle will drop the pH even further. They are ok to 8.4 and probably more.

A lot of municipal tap waters are buffered to a pH of 8.6 to keep lead water pipes from dissolving. If that is gradually added to an aquarium, it will almost always drop some. Since you are as conscientious with the pH test kit or meter as it seems, measure the tank at a water change and again a couple of days later.

Also, what are you using to drop the pH? The more stuff we put in the water the more potential for complications. Some pH dropping chemicals have phosphorus in them too and that can encourage lit tanks to have an algae explosion. "If it ain't broke, don't fix it." ;)

Perfect low ammonia" So it is zero. Right? :)

If ammonia or nitrites are anything, that may be a temporary part of new nitrogen cycles developing or of a tank adjusting to the addition of fish - which can cause ammonia and nitrite spikes all over again. But that should not be the regular state of affairs. Getting close to 20 PPM of nitrates is asking for trouble too. Relatively high levels of fishy waste products also inhibit immune systems and prevent healing.

By the way, you will get a quicker response on Guppylog (assuming that the server is working) if you submit your thoughts as a diary. None of us figure out (in our first week or two on GL) that logs are for multi-page expositions on the hobby (according to the software and site developers). By then we've read the instructions in the FAQs and Immediate Help. ;)

But in the meantime, if you may not want to hassle with waiting to get voted upon (or of getting voted off of the island) and would like everyone to immediately see what you are asking about. So go with a diary. :)