1. Check the algae levels
As soon as you start taking care of your fish one of the first things you'll notice will be the green, slimy buildup on the walls of the water tank. These should be carefully monitored since excess levels of algae can harm the health of your fish. There is a special kind of fish which thrives on them called Plecostomus. While they aren't exactly pretty they serve an incredibly flexible role in the ecosystem of an aquarium since they love to consume algae and even dead fish. Even if it does sound a little morbid, these little fish are the equivalent of crows in your tank, minus the noise.
2. Keep the glass clean
This is something crucial in keeping the water's environment healthy to your fish, though depending
on the size of the tank using the plecostomus might not be enough. With bigger aquariums you will need a algae scrubber from your local pet shop. You need to do this before you change the water
or before you need to siphon some of it out. When you do this you'll be able to clean it up more efficiently and whatever is left will be food for the plecostomus.
3. Change the water
The key point about looking after fish is to only change about 20% of the water at any given time. Changing too much of it will lower the beneficial bacteria living in the water which the fish need to survive. Aquariums have a precarious balance of microorganisms living in the water which thrive living symbiotically with the fish in the tank. Changing only so much of the water will keep the algae levels in check as well and the walls of the tank clean.
4. Cleaning the gravel
The lower parts of the aquarium will usually collect things like food scraps and fish waste so cleaning it up from time to time is a good idea to keep the water flowing. Having an aerating water pump plugged into the tank will keep the algae and waste at lower levels, however cleaning this can't be avoided. Rinse them lightly when you do a full water change and always remember your fish need some of the bacteria in the tank to survive. Wet vacuums are sold which only pick up lighter stuff from the bottom of a tank, though you won't need them with smaller tanks.
5. Filtration is key
When your cute little fish swim around they excrete waste almost all the time so keeping the water clean is essential to their survival. Fish tanks need at least one filter so the fish can live there without quite literally drowning in toxins. Remember that the algae forming on the filters are beneficial to your fish so you don't have to scrub them off completely while cleaning the filters.