So which filter should you buy?
For the fish, plants, snails, and other creatures kept in an aquarium, the concept of cleanliness is far from ephemeral. And the main condition for maintaining constant cleanliness is filtration. The fate of aquatic organisms largely depends on how correctly you choose the filter and what materials you fill it with. Filters clean aquarium water and come in different shapes and with different functions. The most common are hang-on-back filters, undergravel filters, and canister filters located next to the aquarium.
Filters that are best for plants are also best for the aquarium. The life of aquatic plants depends on water. However, the beautiful plants grown by aquarists survive in a nutrient-rich environment. When water is oversaturated with nutrients, lower plants (algae) begin to thrive and choke out higher plants. No filters are capable of cleaning water from such a nuisance, so the aquarist must regularly perform water changes as a secondary cleaning procedure. If the filter is too small or too weak, water changes will not save you from an algal bloom. Bacteria and microbes in a healthy filter fight pathogenic microbes and algae, making it harder for the latter to reproduce uncontrollably in the water. Bad filters also lead to fish diseases.
Looking into a good pet store today, you will see a wide assortment of filters: miniature and bulky, cheap and expensive, simple and complex, attractive and nondescript. Internal filters are well-suited for lightly stocked aquariums. These filters are not suitable for large aquariums because their performance is low. External mechanical filters with turbine pumps have the highest performance. They effectively clean a large volume of water. Their only drawback is that the strong water current they create has an inhibitory effect on the development of aquarium plants.
For modern, large-volume aquariums containing a significant number of fish and relatively few plants, external biological filters work most effectively; their filtering element is located outside the aquarium with a substrate containing a mass of microorganisms. Not only do they break down organic matter, but they also mechanically clean the water from turbidity. Such filters are especially desirable when keeping fish that are very sensitive to various types of pollution, such as Cichlidae from the Great African Lakes. Do not forget that the main task of filtration is to create a biological balance in your aquarium. If the water in your aquarium often turns cloudy, you should consider how effective your chosen filtration method is.
The ideal filter should be inexpensive, quiet, clean water well and quickly, never break, and be easy to install and clean. It is good if it also helps saturate the water with oxygen and quickly removes dissolved substances. 
The main criteria when purchasing any filter should be its performance and filter media volume. The filter volume and water turnover rate must correspond to the aquarium volume, the size and number of fish (and their appetite), as well as water movement requirements. The filter should not be too powerful, otherwise the fish will feel like they are in a centrifuge, and fry can simply be sucked into the hose by the water jet. If the filter is of insufficient power, it will not cope with debris. It is also worth considering that during operation, the unit’s performance will decrease.
The golden mean in choosing this setup is that the filter should turn over all the water in the aquarium within an hour. In a heavily planted aquarium, pay attention to the water discharge system. It should not cause strong turbulence on the surface. A strong water current created by a directed jet is favorable for fish development, but has an inhibitory effect on plant growth. It contributes to the formation of plaque on leaves and worsens the appearance of plants. Just keep in mind that a weak filter in an aquarium whose length is more than twice its width simply will not cope with mixing the water throughout the entire volume, while a powerful one will do so by creating a strong water movement. If you have a fairly large aquarium, it is worth installing several low-power filters in it instead of one large one, carefully decorating them, each in its own section along the length of the aquarium.
It is believed that pump power should be sufficient to pump the entire volume of the aquarium at least 2 times per hour. But this is provided that you have many plants and few fish. If there are no plants in the body of water, or you pay primary attention to fish and keep plants only as an addition, the entire volume of water must be filtered at least 4 times per hour. And the most important rule: the filter must work continuously, around the clock. If the pump stops for any reason, take action immediately, because when the filter is not washed by water, the microbes decomposing settled dirt are deprived of oxygen supply and die. Then anaerobic microorganisms—those living in an oxygen-free environment that release methane, ammonia, and hydrogen sulfide—take over, and within a couple of hours, the filter turns into a source of toxic substances installed right in your aquarium.
A single type of filtration is not enough. All aquariums for keeping fish must have biological filtration. Although chemical filtration can remove ammonia under limited circumstances, it is not a general solution. Typical filters perform some or all of the three types of filtration sequentially. Mechanical filtration usually comes first (called a pre-filter), trapping particles that could clog the subsequent stages of filter media. This is followed by the biological filter, accompanied by a chemical filtration section (if present). Whether chemical filtration is truly useful depends on the purpose. It can be useful for removing leftover fish medication after its effectiveness has worn out. They can also remove elements necessary for plant growth. If you do not have a serious reason to believe your circumstances require chemical filtration, avoid it.
Modern industry offers a huge variety of filters to choose from, and there are even those that combine two functions at once: a filter and an aerator. But it is better to purchase these mechanisms separately so that the harmony in the aquarium does not depend on a single unit. An internal filter is convenient to clean, but the design itself takes up usable space in the aquarium. With heavy pollution, such an internal filter cannot cope with the problem and is less effective compared to an external filter. An external filter needs to be cleaned much less often than an internal one, but the process itself is quite labor-intensive; disadvantages of the filter also include its high cost.
Every manufacturer has its own sets of various fittings, shut-off valves, T-shaped bent tubes, siphons, etc. For cleaning the filter canisters themselves, so-called quick-disconnect couplings with two shut-off valves are especially convenient; the water flow is stopped, and the canister is removed for cleaning without needing to disconnect all the parts at all. Intake filters with an integrated heating system and a built-in electronic thermal sensor have also been developed. When using such filters, heating with regular appliances is not required. These electrical external filters, which are quite large in size, can be used for chemical, mechanical, and combined water purification. When choosing a filter, watch videos from well-known manufacturers, and you will get answers to many questions.
No matter what the filter looks like, it cannot replace regular partial water changes.
So which filter should you buy? External vs internal filters.
Biofilter media. Rational filling of a biofilter. 
