Large water change in the aquarium.
In aquarium fish keeping, a complete replacement of old water is done extremely rarely. Even in the event of a massive fish die-off, it is not completely replaced. Water is completely changed in exceptional cases: introduction of unwanted microorganisms, appearance of fungal mucus, rapid water blooms that do not stop with temporary shading of the aquarium, and heavy substrate contamination. Plants suffer from a complete water change: discoloration and premature death of leaves occur.
When doing a complete water change, you must be sure that the new water meets all the hydrochemical parameters required for the existing fish species. The benefits of water changes, 
must be balanced against the level of stress that can be caused by a sudden change in water parameters. If the water in the aquarium has close pH, GH, and KH levels to the corresponding parameters of tap water, then replacing 50% (or more) of the water at a time will not cause stress to the fish. On the other hand, if the pH in the aquarium is (for example) 6.3, while the replacement water has a pH of 7.5, replacing 50% of the water at once will significantly change the pH in the aquarium (perhaps even by more than 50% depending on the buffering capacity of the water). This will lead to fish stress or even death.
Therefore, it is necessary that the water parameters in the aquarium are close to the parameters of the replacement water. Provided that your tap water meets optimal standards. If it is full of phosphates, you need to think about various ways of preparing water. Often it is necessary to change the water chemistry, for example, increase hardness. Only then is it possible to replace large volumes of water in a short time. Moreover, even a partial water change should not abruptly change its gas or salt composition.
If the aquarium is biologically populated correctly, plants, fish, and bacteria in the soil and water can replace a good filter. The opinion common among novice aquarists about the need for frequent large water changes as a mandatory condition for the normal keeping of exotic fish is deeply mistaken. Frequent replacement of a large amount of water in the aquarium can cause disease and even death of the fish.
By the way, those hobbyists who securely cover the aquarium from above with a cover glass act correctly. It not only protects the aquarium from room dust (which is always abundant in the air and is one of the causes of an unpleasant bacterial film on the surface of the aquarium water), but also prevents heavy water evaporation, and prevents fish and other aquarium animals, many of which tend to jump or crawl out, from dying. The influx of fresh water usually causes an increase in activity in fish.
Super water change in the aquarium.
Super water change implies a large water change in one go, but in several steps. If you drain 80% of the water, i.e., 20% of the volume remains in the aquarium, and then fill the aquarium up to 40% of the volume, this will leave half of the initial concentration. In a planted aquarium, due to the thick substrate layer and complex relief, draining 80% leaving 8-10 cm of water is unrealistic, so let’s adopt the scheme: -30+60> -30+60> -30+100%. The minus in this formula means draining water to leave this percentage, and plus means filling up to that percentage of the aquarium volume (or written otherwise > -2/3> +1/3> -1/3> +1/3> -1/3> +100%), leaving ~8% of the initial concentration.
Option two: The first drain leaving 1/3 and subsequent filling of the aquarium to 2/3 (60%) of the volume will leave 50% of the initial concentration, and then each subsequent drain to 1/3 and fill to 2/3 of the volume will also leave 50%, ultimately the scheme -30+60> -30+100% (-2/3> +1/3> -1/3> +100%) will leave ~16% concentration.
Two such “super water changes” a week, and ammonium NH4 or PO4, for example, will drop from 2.0 mg/L to 0.013 mg/L. To get a reduction in concentration to 6.25% of the original using conventional 50% volume changes, it will take 4 changes and 2 volumes of water. The most important factor is that when changing twice a week according to the scheme > –30+60> -30+60> -30+100%, such a radical decrease in the level of algae stimulants (organics, ammonium, nitrite, phosphate) will take only a week instead of two weeks with 50% changes. Algae will recede much faster.
For example, for a 90x45x45 cm aquarium (~180 liters), the scheme – 30+60> -30+60> – 30+100% (residual substances 8%) in practice looks like this: 1] drain 120 liters, 2] fill 60 liters, 3] drain 60 liters, 4] fill 60 liters, 5] drain 60 liters, 6] fill 120 liters. One step less – 30+60> -30+100% (residual 16%) equals > drain 120 liters, fill 60 liters, drain 60 liters, fill 120 liters. To avoid confusion, think of this scheme as: drain a lot of water –> make one/two “small” changes –> and fill the aquarium completely.
Of course, you need to do such a “Super water change” using a Python water changer, and pour in the dechlorinator at the very end based on the first change, i.e., 2/3 of the aquarium volume. The most inefficient water change method is a temporary “flow-through” system, where two hoses are inserted into the aquarium, and water is drained and filled simultaneously.
Using an internal filter for water changes.
Every aquarist faces one of the necessary procedures related to landscaping and maintaining comfortable conditions for their pets and plants — water replacement or partial water change. Everything seems simple — scooped out water, poured it out, added new. But this method is mostly suitable for open or freestanding aquariums, where access to the top is open for easy maintenance. But if the aquarium is located in a furniture niche or somewhere else where access to the surface is limited, problems arise. How do we cope with this?
Everyone at least once in their life (well, maybe not everyone) has siphoned gasoline (or some other liquid from various containers) using a flexible hose and the physical law of pressure difference. Let’s remember how it’s done: inserted the hose into the container from which the liquid needs to be drained (it must be higher than the container into which it will be drained), and “sucked” with your mouth or a special bulb to drain. Everything is simple… but, agree, inconvenient. Beginners doing this procedure for the first time risk swallowing liquid, which is not a pleasant sensation. But you and I are not motorists, and we have some makeshift tools with which we will perform this procedure, or rather replace part of the water in our aquarium.
You all certainly know how it works: an electric motor rotates the pump blades, which drives a stream of water through the filter. Why not make it perform additional work? Let’s use it to drain water. In this case, we have an aquarium (60 liters) built into a furniture wall, the distance from the lid to the top shelf is about 15 cm. Yes, maintenance is inconvenient, but there is nowhere else to put it. Then let’s begin. For this we will need: a filter, a flexible hose (the hose used in air conditioners for condensate drainage is ideal, I have one left over from installing the latter; if you don’t have one, look at the market or in a store).
A sponge is put on the end of the hose lowered into the aquarium to prevent hydrobionts from being sucked in. The hose is passed through a drain elbow (from a washing machine) so that the hose does not kink. A valve is cut into the hose through fittings; at any time you can shut off the water, both filling and draining. After the required amount of water is drained, the shower hose is unscrewed from the mixer and replaced with a plastic fitting onto which a silicone hose is put.
Let’s start. We take the filter out of the aquarium and put a flexible hose on the nozzle or adapter. We lower the filter into the aquarium, and the other end of the hose into the drainage container. In this case, it does not matter at all where this container is located — above or below the aquarium — we have mechanics, not a pressure difference. It all depends on the length of the hose. Turn on the filter and drain the water. Everything is very simple. Then, to fill the water into the aquarium, we perform the reverse procedure, or rather, lower the filter into the container with prepared water, and the other end of the hose into the aquarium, preferably against one of the walls so that the water flows down the plane.
But there is one catch here. As you know, the filter contains a filter element in the form of a sponge, which is home to many microorganisms beneficial to the aquarium, the loss of which is undesirable. Before draining water from the aquarium, it is recommended to remove the filter element, thereby preventing the drainage of microorganisms along with the replaced water. Removing the filter element is also necessary in order to pump out the prepared water from the container where it is located as much as possible. To do this, the filter also has a tube through which water flows from the filter element through the pump into the aquarium. If you place the filter together with the tube at the bottom of the water reservoir, you will achieve maximum effect — almost all the water will be pumped.
What water to pour during changes.
Regular partial water change in the aquarium. 
