Q: Hi. May I know if it is possible to keep ornamental fish in a planted tank without changing water and only by toping up due to evaporation? If so, how?
A: Dear Wei Chao,
The answer is YES and NO.
There are many other ions that is undesirable other than nitrogen based ions (i.e. Ammonia, Nitrite, Nitrate, etc). Therefore, there have to be ways to remove these ions from the water, of course, one way is the use of plants. There are also important ions that is desirable and needs to be added to the tank so as to maintain a balanced “ecosystem”.
YES. Assuming that your tank loading of fish and plants are well-balanced (which can be complicated, depending on how many fishes you have, how many plants you have, how much fertilizer and usable light you are giving, etc), you can keep topping up the tank with water PROVIDED you are also trimming away the overgrown plants. The overgrown plant is one mode of removing unwanted “materials” from the tank, thus balancing the whole system.
NO. if you tank is already not well balanced, i.e. the number of fish is too much or too little, plants are not growing well due to the reasons mentioned above, then the tank will not be able to sustain just by topping up water.