I'm still experimenting with gelatine in the DIY CO2 recipes (posted on the old and new Aquabotanic board).
As I was asked privately about how to do the mix, I figured I'd post it here along with my current tests and as-of-this-minute theories of what works and why. (Standard disclaimer here, use this information at your own risk! It is possible to kill your fish with DIY CO2, start with small volume bottles and small mixes first.)
I am using a version of the Knox gelatine "Knox Blox" recipe posted on the back of the box, to provide the sugar in the DIY CO2 recipes. I am using 1/2 of the gelatine in a bottle, so there is room for 4 or 5 cups of water. The Knox Blox is altered to use 3 cups of water and 2 cups of sugar substituted for the fruit juice in the recipe on the box. That means that the total mix contains 4 cups of water, 2 cups of sugar, and 4 packets of gelatine. Chill overnight either in a pan to be cut into cubes or in the bottle that you use as the generator. Allow it to come to room temp and fill with warm water, allowing 1 to 4 inches of space at the top. To that I add 1/4 teaspoon of yeast, then maybe baking soda, salt, and/or yeast nutrient. Actually the best thing to do is to add the yeast to the warm water and let it sit for a good while (10 to 30 min), then mix the rest. Sometimes I add a 1/4 teaspoon of sugar to the water too.
I've been testing gelatine with regular bread yeast, champagne yeast, and ale yeast along with various combinations of baking soda, salt, and "yeast nutrient" from the home brew store.
I've used some pretty scattershot methods and there are still many very basic combinations that I have not tried. The Hagen/Nutrafin CO2 System seems to use baking soda and something that I thought was salt, so I began there using gelatine for the sugar. Then I tested different yeasts and some yeast nutrient. As this has all taken place in my fish tanks, each test has taken about a month or sometimes less, so this is very slow. Plus, any trip out of town means no data. So, it's been slow.
I think gelatine in cubes works better than solid as it gives more surface area for reaction but the sugar is still slow to release. The protein in the gelatine is probably good for the yeast too and may even substitute for the yeast nutrient used by the brewers. The nutrient that I got has several phosphates and a lot of vitamins added.
Baking yeast (fresh or refrigerated if open) with 2 teaspoons of baking soda and 1/2 the Knox blox recipe in cubes in a 2 liter (actually 64 ounce) bottle would last about a month at a reasonable rate. Baking yeast seems to really benefit from the baking soda, I suppose the pH effect is important.
Champagne yeast with 1/2 recipe gelatine cubes, 2 teaspoons of baking soda, 1 teaspoon yeast nutrient foamed over really badly. Too much nutrient? Champagne yeast with solid gelatine was likewise not a good rate, but I didn't add any yeast nutrient, it was a bit foamy already, no need to aggravate that. Even though the bubble rate is not high, it keeps the tank pH at the usual level that I am able to reach with CO2 with my filter set-up (the HOB blows off a lot of CO2 so I can't get much lower regardless of the input.) I will open the bottle to stir it up if I have to later on.
Ale yeast with the same 1/2 recipe in solid form in the bottle performed really badly so I added yeast nutrient, it has foamed but not too bad (I left a lot of room in the top) The rate isn't improved. I'll stir that soon also to see if better contact improves the rate. There is a thick sludge of yeast stuff on the top of the gelatine, is that good or bad, I don't know.
Meanwhile, not using gelatine, a Hagen bottle with 1/4 teaspoon ale yeast, 1 teaspoon baking soda, 1/2 cup sugar and 1/4 teaspoon nutrient is doing really well. It was slow to start and the two bottles are behaving differently so I can't say that I know why just yet. One had remnants of bread yeast in it (the better one), the other had remnants of Hagen yeast in it (the lesser one). An earlier test of a Hagen bottle with bread yeast and baking soda with sugar was poor.
I had decided that the brewer's yeasts didn't like baking soda, but then this one proved me wrong. I need to start this one over and take better notes.
So, that's an update on the gelatine tests, and the non-gelatine tests, too.