First off you need to find out what types of fish are compatable with the fish you have currently or are wanting to buy. Example/ you do not want to have a Oscar and a guppy in the same tank as the oscar will eat the guppy in a matter of seconds. Other things to consider is if you stay with large or small fish what kind of algae eater to use. Example/ for guppies use a glass shrimp and a corydora as they tend to be less aggressive and do not like to eat other fish because they are bottom scavengers. For a larger fish like a oscar or a piranha you would use a plecostomus or a golden algae eater as they are also a meat eater and will often clean the sides of your large fish but they will also sometimes kill them or will definitely eat their young despite of their sucker mouths. Secondly before you finalize your decision on what type of fish, make sure you have the compatable aquarium for it, Example/ guppies, tetras, platies, swordtail, silver dollars, etc. can live happily in a 10 gallon aquarium and some times smaller depending on the amount of fish you have. On the other hand a oscar, piranha, certain freshwater sharks, ect. that get fairly large need at the least a 30 gal. aquarium if you only have 1 fish. If you have 2 or more of the large breed it is best to go with a 55 gal. Also before you buy online make sure to ask if the fish you are buying are free from diseases like ICK, mites, parasitic worms, fungus, and external / internal parasites. Also be sure to ask if they are healthy. Once you receive the fish, look at the fish in the bag. if you see white spots on the body, tailfins, or dorsalfins, or tiny worms swimming in the bag do not add it to your aquarium. Also if the fish is highly discolored from its natural look or if the eyes have a thick white film covering them. That would give an indication of a internal parasite or being near death. Be sure to clean your tank regularly, it is recommended once a month but I do not clean mine but about every 4 months. If you have the right bottom cleaners with the right fish and the biology of the tank is well, it tends to take care of itself for a long period of time. When I do clean mine it is only usually a 25% water change which is just enough to clean what nitrate ( waste ) there is in the gravel at the bottom. With a larger breed of fish you will have to clean more often as they tend to produce more waste. Speaking for the biology of the aquarium it is ideal to keep everything as low as possible except your PH. The ideal PH is 7.0 for a freshwater aquarium. 6.8 - 7.4 is a good area to keep it. Your PH determines on how acidic your water is. For your ammonia, nitrate, and hardness it is best to keep them as low as possible. Although water hardness does not have a great deal of effect on the fish, it does have an affect on the life of your filtration system and other working components for your aquarium. Also when using aeration such as air pumps and bubble stones it is good when you can mimic a current flow of the water. Like their own miniature lake or stream. You will find the fish swimming with the current having fun. Which is very neat to watch. Thank you for reading my helpful tips on finding the right fish for your aquarium and some other things to look for when purchasing them and adding them into your home. I hope some of this information will be useful for your new addition and or current occupants.