On average, I do my high light 10 gallon once a week. 50% water change. If I an trying to make it look real nice and clear for photos, I might change it every 2 days, 50%.
My lowlight 10 gallon with crypts and anubias/ no CO2 I change every 2-3 weeks. Basically when I see the glass is a little dirty.
I have a 5.5 gallon, low light with ferns, mosses, and crypts mainly and I do it every 1-2 weeks. It is a grow out tank for 5 rosy barbs. I know I should change this more but .......
I also an setitng up a 15 that i had laying around (as we speak actually )and it will get the same treatment as my oter high light tank.
All of my tanks are low to moderate fish loads. High light 10: 7 neon tetras, 5 amano shrimp, 5 ghost shrimp (a couple of each shrimp will go to the 15 gallon) low light 10: 3 neons, 2 bolden barbs, 2 ottos, 1 zebra danio and one spotted cory.
In many cases I see your changes depending of light quantity, and this a support for the idea that less light implies little grow and, so, less CO2 and nutrient demands furnished by new water. If you change weekly the CO2 enriched tanks is mainly due to the micro-nutrients, isn't it?
Near as I can tell,
The reason for fewer water changes as a function of lower light/CO2, is one based on rate.
At lower growth rates, the tank(bacteria and plants) have more time to reminerlized these organic fractions and small low residual lecvels are still able to supply the plants with their growth needs that a greatly slowed by less light/CO2 etc.
As you increase the growth rate of the tank, you need more and more inorganic sources for nutrients.
Water changes remove the organic fraction and also organic accreation or mulm etc. Algae like the organic fraction at high light.
At low light and non CO2 cases, the algae have less energy to drive this process.
All my tanks get 50% weekly water changes except the single low light setup. The low light setup used to get changed about once every 3-4 months, but I'm going to lengthen the time to six month intervals...old habits are hard to break. :wink:
I guess it also depends on the balance in a tank, in my case, the moderate-high fish load, automated feeding and plenty of plants seem to do well at maintaining levels constant in the tank. Nitrates are a stable 5ppm all the time, other levels also very constant, I add no liquid fertilizers, just tabs in the substrate every 4-6 months. As far as fish health goes, I've lost far more fish in my high light tanks than low light, so I'm not sure that everything we do to put the plants in turbo mode really does the fish any good in the long run.