Hey Peter, I don't see why you need to go bonkers here. Who says you need high fish load? Who said you don't need nutrients? Who says to trade the latter for the former?
The only hard fast rule for a non-enriched tanks is that you keep the lighting low. End of story.
The nutrient requirements are low. So what? Add low quantities of nutrients. I mean, doesn't that sound rational? As long as the lighting is kept reasonable you may not need to dose more than a couple times per month if at all, but having high fish loads is asking for trouble, no matter what type of tank you have.
In Barr's non-CO2 method it is assumed that the organic waste production, which will be at much lower quantities than in a CO2 enriched tank, will be the main source of plant nutrients. Now, of course, this includes fish waste and decaying food/plant matter. This doesn't mean that you need to go crazy and dump a thousand fish in the tank.
Adding some inorganic nutrients though improves performance because it's accessible immediately. The dosing scheme goes something like:
Add roughly 1/8 teaspoon of KNO3 and 1/32 teaspoon of KH2PO4 per 20G respectively once a week or two. If you're using commercial ferts, you can just dose per bottle instructions (assuming it's an NPK mix).
While trace mixes can be added, SeaChem Equlibrium is suggested as an alternative, since it has Fe and Mn as well as Ca/K/Mg/SO4. But you can add these if you already have them.
Standard dosage for Equilibrium is 1/4 teaspoon per 20 gal tank once every week or two.
So that's it. No need to go bonkers. Water changes are optional in this method. This helps to control BBA and other CO2 related algae. Just top up when needed. Again, if you keep stocking rates reasonable then the plants absorb the toxic products and recycle the waste. This method cannot possibly work in a non-planted tank, obviously, since the waste simply builds up and is only controlled to a limited extent by filtration. It also cannot work in a CO2 enriched tanks because the rate of waste production is 3X-10X higher.
You need to think about what's happening conceptually. Low light lowers CO2 and nutrient demand. Low CO2 reduces the metabolism so that waste production is dramatically lowered. Since it takes some time to break down the organic waste and to convert into nutrient products that the plants can then absorb, supplementation with small quantities of inorganic fertilizers enhance growth performance enough to keep the plants from stalling and to keep them healthy. Healthy plants then detoxify the environment obviating the need to replace large quantities of water. Avoiding large water changes prevents short term fluctuations in the CO2. It is these fluctuations that are implicated in CO2 related algae.
Again, you don't need to freak out about water changes. You can do them if you feel the need but if you do, try to perform them after the photoperiod, or at least let the water degas fro a day or so in order to maintain the same CO2 concentration in the new water as that in the tank. This reduces the difference in CO2 levels. Having plants in the tank is totally different than having an unplanted tank. Plants clean and oxygenate every environment they are in. That's why grass grows greener over a septic tank and why the weeds growing out of a cow pie are healthier than the ones growing away from it. That's their job. Without plants the world would be a very messy place, so one need not be aghast at the concept of eliminating water changes because the plants are changing the water for you.
Cheers,
