How to prepare your aquarium for a long absence.
Before a long trip, try not to buy new fish and plants (to avoid introducing infections), and avoid purchasing fry or very young fish that still have a lot of growing to do. Do not overfeed your fish right before leaving “just in case.” They won’t eat more than they need anyway, and the excess will only decompose and ruin the water. It is a good idea to siphon the substrate and replace 1/3 of the water with fresh water before you leave. If you will be gone for a long time (3-4 weeks), it also doesn’t hurt to wash the filters and rinse the mechanical filter sponge; dirt accumulation in it reduces the overall performance of the filtration system.
First, you need to start siphoning the bottom of your aquarium. It is advisable to siphon it in stages rather than all at once, so it’s best to start preparing the aquarium a few days or even a week before your departure. Our goal is to siphon the substrate as thoroughly as possible. For example, on Monday we siphon 30% of the bottom, on Wednesday another 30%, and on Friday the remaining 40% of the aquarium bottom area. Water changes are performed each time. At the same time, clean the decorations, plants, and glass.
The aquarium filter should be washed and cleaned the day before departure, or preferably on the day of departure if time permits.
Feeding fish and plants. 3-5 days before departure, aquarium inhabitants should be put on a light diet. Slightly reduce the amount of food given and exclude foods that heavily pollute the water from the diet. Do not give organic-based fertilizers to plants before leaving, again to keep the water as clean as possible.
It is better to set the water temperature 1-2 degrees lower than usual before leaving; this will slightly slow down all biological processes in the aquarium. However, if you already keep your fish at the lower threshold of their acceptable temperatures, you should not lower it any further.
Upon returning from vacation — first, you should rinse the filter, siphon the substrate, and change about 20% of the water, and only then feed the fish, but just a little. Keep in mind that all actions must be done gradually; for fish, a sudden change in their environment for the better is just as harmful as a sudden deterioration.
Here are a few common mistakes made by beginner aquarists that you should avoid.
The first mistake is turning off aeration at night. For normal biofiltration, bacteria need oxygen, which they receive along with water that is saturated with oxygen through aeration. In addition, oxygen saturation occurs when the water surface contacts the air, and plants also release oxygen during photosynthesis. However, at night plants stop releasing oxygen, and it is at night that many people turn off the air pump to avoid operating noise. Thus, out of three oxygen sources, only one remains. As a result, the total amount of oxygen dissolved in the water decreases, which affects biofiltration intensity, and fish may lack oxygen if stocked heavily. Bottom line: aeration must run 24/7.
The second mistake is turning off the filter at night. Some aquarists turn it off at night to avoid noise, stopping the oxygen supply to the filter. The aquarium filter must run continuously! Turning it off (even temporarily) is both pointless and harmful. Why must the filter be on constantly, 24 hours a day? The thing is, during the operation of both internal and external filters, those very beneficial bacteria that carry out biological filtration settle on its sponge or ceramic media. And since bacteria need oxygen to survive, the filter must constantly circulate aquarium water through itself.
Keep in mind that if you turned off the filter for more than 1-2 hours, the filter media should be rinsed before turning it back on. By turning off the filter, you deprived the bacteria colonies involved in biofiltration of oxygen, causing them to turn into a decomposing organic mass. If the media is not rinsed, once the filter is turned back on, all toxic compounds, including intermediate ones (the most dangerous), will enter the aquarium from the filter, disrupting water parameters (primarily pH), which in turn can trigger a mass die-off of fish and consequently cause the aquarium water to turn cloudy and the biological balance to collapse.
Basic aquarium maintenance measures.
Organic matter. Use of peat. Humic acids, Tannins. 
