Now that your aquarium is entirely setup and has completed cycling it is time to start adding fish. Although it is tempting to go to your local fish store and start buying the most colorful fish you see, there are a few things I recommend you consider first.
Within the fish keeping hobby, the general guideline for keeping fish is having one gallon of water per inch of fish. While that may sound to you like you can throw 20 fish in your 5 gallon tank, realize first that the fish you see in the store are still young and will grow bigger. This is commonly the reason I see people overstock their aquarium. They hear this very general guideline and instead of considering the length the fish will be, they instead look at the size the fish currently is and drastically overstock their tank. It is best to ask your local fish store or search online for the maximum size the fish you desire will be and use that to consider your stocking options. Contrary to popular belief fish do not “grow to the size of their tank.” What is actually happening is the fish is being stunted which can lead to a higher risk of disease and increased aggression. You should avoid this, as it will save you time, energy, and money long term.
Despite the gallon of water per inch of fish rule working for most fish, another thing to consider is the activity level of the fish. Freshwater sharks and fish such as danios (just to name a few) are very active fish that like to move constantly. These fish need extra room to swim so it is best to give them additional space where they won’t feel quite so contained. Without this extra space the fish will “pace” the smaller tank and likely become stressed out leaving it prone to diseases such as ich. One option that can help is getting a shorter, but wider aquarium such as a 20 long instead of the standard 20 gallon aquarium. This will provide a sufficient “footprint” for these fish to swim and prevent any ill effects.
People new to the hobby may not know, but some fish are “cleaner” than others. Take goldfish for example, with these fish the general recommendation is to have 10 gallons of water per fish because of their final size and relatively “dirty” existence. This also factors into your stocking options rather importantly. On the opposite side you have corydoras which will weigh a little less heavily in your stocking decisions because they will help to clean up some of the excess waste created by your other fish.
Not every fish keeper has the same level of experience and this will weigh heavily in your stocking options. Some fish are very sensitive to certain parameters and these fish are best to avoid for new hobbyists. This could be because of a specific food requirement, a habitable pH level that is hard to maintain, or an increased maintenance schedule in order to keep your aquarium absolutely pristine. Regardless of the reason, any reputable local fish store should let you know which fish may prove to be difficult to keep as well as the fish that are best to start off with. Some beginner fish include mollies, platies, guppies, swordtails, corydoras, and bettas. These fish will provide you with a successful first experience that will give you the knowledge needed to tackle some of the more picky options.
Important to consider as well is the compatibility of the fish you are thinking of purchasing. Generally you want fish that prefer similar temperatures and pHs and stay roughly the same size. This is vital, since fish that are significantly larger than others tend to dominate their tank mates at food time and if the size difference is large enough they may even begin to eat the smaller ones. Additionally if the pH or temperature requirements are different from one fish to another you may end up with a fish that is frequently stressed and very prone to disease which will ultimately spread throughout the aquarium. Talking to your local fish store should help you make these decisions with ease but searching for stocking suggestions online will help you create a plan that you are happy with first and make the process go faster.