A few months ago, I remember reading some exchanges between Lola_Granola & Captain Findus Goldfish about the use of agar agar as a substitute for gelatin when making gel food. About a month and a half ago, I PM'd CFF about this and to ask for her suggestions on how much agar agar to make. I'm happy to report here that using agar agar as a gelatin substitute has been a great success. I highly encourage using it.
To back up a little bit, a lot of us here, after some time using commercially bought foods, have also made our own gel food, to supplement their pellet diets or sometimes to replace it entirely. The reasons are manyfold. First, it can be cheaper to make gel food than to buy pellets, especially because pellets need to be replaced every half year or so. Secondly, when you make your own gel food, you have total control over what goes into the good, as well as the exact composition of your food. Lastly, as many goldfish keepers have found to their dismay, sometimes the commercially available staple food causes issues with floatiness, etc. Gel food certainly helps with alleviating this problem!
So why agar agar, and what the heck is agar agar? Essentially, agar agar is a natural product derived from seaweed (red algae) and has some very similar properties as gelatin, which is animal collagen. The problem with gelatin based gel food is that sometimes it does not help with the floatiness, but actually can make it worse. I had this problem when Ulquiorra became almost permanently stuck butt up at the surface and had to struggle to get down for food.
Enter agar agar. As a seaweed type product, one can argue that it is a lot more goldfish friendly than animal collagen. Agar agar is also 80% fiber, so it is also a great natural laxative. So far, after over a month of switching to agar agar, I can tell you that my fish are totally happy. Ulquiorra's issues have largely disappeared, and my fish poop very regularly lol. I can't recommend this product enough.
You can buy agar agar at places like Whole Foods or any Asian grocery store. You can substitute agar in the exact same amount as what was called for with gelatin in your recipe. Agar agar actually solidifies a bit more strongly than gelatin, so your food will hold even better. Or you can decrease the amount used to suit your preference.
In any case, for those of you who are using agar agar or have used it, please add your opinion. Thanks!