Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I do not think that is a problem at all! Very good way to get nutrients to the plant-only tanks, and 'treated' water to the aquariums with fish. It is almost like adding a planted refugium to the tank, and getting the advantages of a larger water volume to dilute the wastes, and the removal abilities of the plants.
A few cautions, though:
Do not add medicated water to the plant tanks.
a) whatever you are medicating the fish for can get into the plant tank, and might then spread to your other tanks as you share water changes.
b) the medication has a certain dose and timing to properly kill the disease or parasite. To remove the water with its load of pathogens to a non-treated tank halfway through a treatment is to risk encouraging antibiotic resistant diseases.
Test the water parameters (GH, KH, TDS, pH) when you are adding water back to the tanks with livestock. Fish prefer these parameters to be stable, with not a sudden big change. If the GH, KH and TDS drop by 10% or rise by 15% these are safe for even the more delicate fish. A greater change than this may not be safe.
Fish food is high in N, P, and most micros.
Fish food is low in K, Fe, Ca and Mg.
If the water in the fish tank has high NO3 from fish food, then your planted tanks may still need you to add potassium and iron. Monitor the GH and make sure it is not lower than about 3 German degrees of hardness, or you can dose a GH booster like Seachem Equilibrium (contains potassium, too) just to be sure the plants will get enough Ca and Mg.