In the course of time unwanted substances may accumulate in your aquarium water. Your filter will remove nitrogen compounds, such as ammonium and nitrite and your plants will take algae promoting phosphates from the water. Your water conditioner may have bound any heavy metals present. So, why do you still need to change the water?
Along with the substances mentioned above there are quite a few more which accumulate through fish excrements. Some of them colour the water yellowish. But the partial water change (you don’t change the whole water, but just a part of it) does much more: It can be compared with the airing of a room. Although the oxygen content doesn’t fall below the natural value of 21%, we feel better, fresher and more lively after the airing. You can observe something similar with fish which even often spawn after a substantial partial water change!
The partial water change is the most important maintenance step, and it should be carried out together with the cleaning of the substrate. Siphon about 1/3 of the aquarium water with a hose. Instead of just sucking off clear water please connect the hose with a gravel cleaner ( JBL AquaEx Set 45-70 ). Go with its suction piece gradually through the aquarium ground and siphon the debris off (similar to a vacuum cleaner).
Subsequently the sucked off water need to be replaced by tap water and with the water conditioner JBL Biotopol you can remove the heavy metals and neutralise any chlorine contained in the water.
Only in rare cases would your tap water be suitable for your aquarium fish in an unchanged condition. The tap water needs to be “conditioned”. Due to your water pipes it might well contain heavy metals, such as copper, lead and zinc. For us humans the quantities might mostly be harmless, but for our aquarium dwellers they can pose a serious problem. Furthermore chlorine is added to our tap water to sterilise it and thus make it drinkable. Chlorine is fatal for fish and invertebrates. Therefore it needs to be neutralised by a water conditioner.