When raising young fish, it is a relatively good idea to occasionally do large water changes to remove waste products from the aquarium, but this is not the commonly given reason for the large water changes; the more common belief is that fish release growth stunting hormones/pheromones, but I have been unable to find any scientific proof to back it up. It is true that fish release a growth hormone (GH) which stimulates growth, but they can also become resistant to the hormone if they are already stunted, for example, because the competition for food is too high. There are also other factors that affect growth, including higher nitrate concentrations. Of course, any ammonia and nitrite are very high on the list, but this should not be a worry as neither should be present in an aquarium.
Before starting on the water change, I check the water parameters. I know that my tap water is quite hard, and the results below show lower water hardness, lower pH and higher nitrates. This means that my tank water hardness has drifted downwards from my tap water and not all nitrates are being used up, which indicates that I need to refill the tank very slowly after the water change so that the fish have time to adjust to the change in water parameters and do not go into osmotic shock.
Parameters before the water change:
- KH: 8 ° (143 ppm)
- GH: 16 (286 ppm)
- ammonia: 0 ppm
- nitrite: 0 ppm
- nitrate: 20 – 30 ppm
- pH: 7.5
As can be seen in the photo, the tank is really rather overgrown with Pistia stratiotes (water lettuce), so I regularly have a bag or so of water lettuce to sell. If you would like some, reply to the post or contact me for more information!
 John Colt, Robert Ludwig, George Tchobanoglous, Joseph J. Cech Jr. (1981), “The effects of nitrite on the short-term growth and survival of channel catfish, Ictalurus punctatus”. Aquaculture , Volume 24: 111–122
 Jane Francesa, Geoff L Allana, Barbara F Nowak (1 April 1998), “The effects of nitrite on the short-term growth of silver perch (Bidyanus bidyanus)”. Aquaculture , Volume 163 (Issues 1–2): 63–72