I'm impressed that you can keep a fish alive as long as a month. I've done all of that and can barely keep them alive for a week. The only fish that I could keep where the Baja sharks and Angelfish. For the most part, I give credit to my mom for keeping them alive because I've never been able to duplicated it on my ow. So you have my sympathy. I've been looking into aquaponics to keep fish, maybe that will work for you. Good luck!

Part of the reason I'm scared of getting a pet fish is that I'm scared they'll die despite my best efforts. When I was in middle school, I won some fish in my biology class so I got to take them home. I was quite happy with them, but they didn't last long. Two weeks after I'd taken them home, they were all dead and I was devastated.

This reminds me of a ten gallon tank my family used to own. We never understood why, but no matter what fish was put in it, they were almost guaranteed to die. I hate to say it was 'cursed' or something silly like that, but the type of tank obviously matters. Some tanks just don't work.
Another tank I owned, a twenty gallon one, kept the fish alive quite well. We had a pair of danio fish that lived for a few years, and the suckerfish lived even longer. We also had a couple tiger barbs that lasted for a year or two as well. You have to make sure you change the tank water regularly and that there are no sudden changes in temperature in the water. Other than that, that's all I know about keeping fish.

The thought comes to mind that perhaps we stress our fish by over-cleaning the aquarium. People think that the tap water has to be perfect, the temperature has to be within a certain ideal range, and the tank itself has to be kept scrubbed to a glimmer at all times. To me, this is taking a nice, relaxing hobby and turning it into a nightmare. I discovered all on my own, that keeping your tank so perfect often results in exactly what you are trying to prevent with all that perfection... fish death. Most fish can handle a little algae in the tank, some even enjoy nibbling on it for a tasty and healthy treat. Changing the water too often, disturbs and even destroys the beneficial bacteria that builds up over time. This bacteria helps keep the tank's environment healthier and without it the fish are stressed. Fish can also get stressed if we are constantly reaching into the tank to do stuff. Stressed fish are unhealthy fish, and unhealthy fish are prone to every little ailment that comes along. Remember, fish in the wild do not have people constantly looming over them and reaching into the water and continually cleaning stuff. How would you like it if someone broke into your home, stole your favorite toy, and then brought it back looking completely different? It would stress you out. It's the same with pet fish. Leave them alone more, and allow them to get accustomed to their new home. Enjoy them but don't stress them out by always having to fuss with the tank.

From my experience with keeping fishes, it's not something for everyone to do because your environment plays a big role in how they are affected and how long they stay alive.

The kind of fish you want to keep would determine the kind of water you use. Whatever that kills a fish is in the water.