Hello. I'm a newbie here too (though I apparently joined the site several years back, probably to ask a question about Green frogs in a backyard pond).
Anyway, I had a big Grey tadpole adventure this past summer, raising nearly 60 rescued tadpoles (long story). I might be able to give some pointers, as I was pretty successful. All of them morphed successfully (except one tad who was the victim of a siphoning accident). I raised them all to pretty decent sizes, and released (I believe) 52. I have 6 left that I'm keeping as pets.
In my experience, the babies did not start eating right away. It seemed like it was close to a week after completely absorbing their tails that I started seeing them hunting fruit flies. If you are going to do fruit flies, I would suggest getting some culture medium and culturing supplies, as that many froglets will go through a lot. You can order everything you need online. I also ordered lots of tiny crickets from Ghann's Cricket farm.
I think they probably were getting adequate nutrition in the horse tank. They normally breed in temporary bodies of water, and feed on detritus and algae. In my experience, they developed extremely rapidly once the leg buds appeared, like from barely visible leg buds to being completely morphed in about two weeks.
I never saw any cannibalism in my tads. You might want to keep them a little less densely populated. I had mine divided between two 10 gallon tanks about half way full, with lots of plants, fresh lettuce leaves on the water surface, and a couple feedings a day of high quality fish food.
I used medium sized Kritter Keepers to create special morphing habitats. I did a gravel substrate, shallow at one end and deep at the other, to create a land and a shallow water area. Added a Pothos clipping, some moss for the land area, and a small piece of cork bark. As soon as the tails were absorbed I moved them to ten gallon terrariums set up along similar lines, to make way for continuing waves of new tadpoles ready to morph.
I can post more info, and some pics if you'd like, later on. This system worked very well for me, as I had a basically 100% success rate with it.
This is basically my first post here, and I'm up too late, but I can post more later if you want more info, or if you have any specific questions.
They really are adorable babies, and I hope you can find a system that works out for you.