Common fish tank problems would be your fish are sick, they're not eating, the fish's eyes are cloudy, just general health concerns of the fish. To fix that or to solve those problems you've got to look a little bit deeper. Let's find the exact cause. Let's see what's wrong with the fish. Is it clamped fins? Is it white spots? So you, the person who's taking care of your fish tank, are going to be the best person to solve your own problems.
What you do is look at the symptoms and then call a store and tell them what the symptoms are, or go to a fish store and try to get the appropriate medicine. And once you've fixed the problem you want to do whatever you can to prevent if from occurring next time. And you want to try to figure out what caused it in the first place. Maybe your tank was overfed. Maybe it was overcrowded. Any of the things that would result in fish being stressed or sick will result in immune systems being compromised, and that's how fish get sick.
It's going to keep happening and happening if you don't adhere to the basic principles of aquarium keeping. Don't overfeed. Don't overcrowd. Pick the right fish. Pick fish that are compatible. Make sure you follow a set photo and night period. You want to have 12 to 13 hours of light and 12 hours or so of darkness. You don't want to keep your lights on all the time or the fish are going to get stressed.
You want to do regular weekly water changes to prevent problems. If you don't feel comfortable testing your own water you can bring water to the aquarium shop. Have them test it for you and tell you what the problems are. Maybe your pH is out of whack, or maybe your temperature is too high or too low. Again, temperature is not something that a fish store can test for because temperature is going to change as you bring your water to the store. But your ammonia, your nitrite, your pH, your hardness, all these things can be tested at an aquarium store and they can tell you if your levels have gone out of whack.
Once you identify the problem then you go about fixing it. If it's water chemistry usually some water changes will get the system running a lot more balanced. You can add bacteria to the system to help speed up the bacterial growth which will keep your ammonia and your nitrite in check. Cleaning the filtration in a manner so that you don't shock the system will be beneficial to curing problems. Basically you want to create a very, very good water environment for your fish. You want your water levels to be perfect so that your fish have every chance for their immune systems to function as they're supposed to.
That being said just look at the symptoms on the fish. The fish will tell you what's wrong with them. If you see that their fins are clamped a lot of times it's ammonia. If there are spots on the fish they have a parasite that can be treated with regular copper or any kind of medication. So you go to the store, tell them the symptoms, and they should be able to give you the advice to cure your problems.