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! But guppies in my experience were the weakest fish of all livebearers I had. They died and died.
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!
Cyprinids are a group of fish very strong. In my opinion some of the strongest small and medium size community aquaria fish. Rasboras, barbs, minnows are cyprinids too.
Gold barbs. I have four of them, I had five but one died stucked in a sponge filter against the aquarium glass wall together with a small electric blue acara. I learned the lesson that sponge filters should be better used for fry only, or if you have them in bigger aquarium keep it away from the sides, front and back of the aquarium, or some fish may get stuck and die there. No matter what everybody say, I don’t keep barbs with aquarium plants. When barbs are small or young they may not eat the plants, but when they mature you’ll start questioning why your plants are getting holes everywhere.
Espei rasboras. Many rasbora species are cheap, and that’s good because they make excellent choices for planted aquariums, and won’t eat plants at all
Tetras. There are many tetras in the hobby, some are excellent for planted aquariums like the ember, blue neons, cardinals, pristella, black phantom, black neons, glowlights(all of this are my favorites for the planted aquarium).
There are bigger tetras too, that are more aggressive. They do well with bigger fish, but they eat plants. Diamonds, Colombian, Buenos Aires, Congo, lemon, etc..