First and most important is that I do 30% water changes weekly, and the tap water here is around 7.2 Ph. And still I found during my experience that some fish are stronger than others and get not sick as some others. But that is relative too. Because it depends if a fish was previously with a weak inmune system where you got it, or if it was a weak specimen, etc. 

Lots of guppies had been raised here in the aquarium. Long ago I read in a saltwater fishing book that fish are like humans, some stronger than others. So you may get confused if the first fish from certain specie died prematurely, but after that the next you got all lived for years! 

Still here are the fish so far I found stronger! 

Betta. They like small places. I have one in a plastic marina breeder modified as a permanent house for her. The water flows constantly, and she had been living eight months there. These fish are tough. As long as the water is clear and in good conditions, the fish will be happy. In their natural environments, they had been found in spits of water waiting for more rain to come to their rescue. 

Angel fish. They never gave me any problems, and I got them when they were tiny specs. 

Silver dollars. A school of five I got when they were very small. Their grow rate is fast!. They can be mixed with cichlids and other fish. But don’t get them if you have a planted aquarium. They are vegetarian piranhas! No plant will survive with them around. And I mean none! 

Gold barbs. A better sentance should be all barbs or cyprinids, because I have espei rasboras. The famous zebra fish too had been almost indestructible. So barbs and cyprinids are in my experience the group of fish with the greatest health of all. The problem with barbs is that they eat plants. So barbs is better to have in an aquarium with sand substrate and no plants, or with Java moss, anubias and ferns because they don’t like those plants.