A bit of a diversion from 'manure in an aquarium', but useful for gardeners...
I did not think blood meal was high carbon.
See page 2 of this link for C:N ratios of some of the materials we are discussing.
Blood meal has high enough N so that it ends up as a fertilizer (N left over) when the composting is complete. There is carbon in it, but when you are making a blend that will compost quickly blood meal is treated as a source of N.
Legume hay (alfalfa, clover) has a little bit of extra N, but not much.
Oat straw (a dried grass commonly fed to horses around here) is so low in N it is practically all C. (Actually HAY is harvested earlier while there is still some nutrition in it, but it is still high C, low N)
I agree that new mown lawn, grass clippings, is very high N. It is almost all water, no structural material. Dry it out and what is left is high N, low C. The dried grasses fed to animals are more mature, and do contain plant structural material so are much higher C than lawn grasses.
I am saying the blend of bedding (carbon) and manure/urine makes a reasonable C:N ratio so it will compost quickly. I understand the high lignin content of most bedding materials does not fully compost as fast a non-woody plant material. There is also the issue of toxics that may be part of the original material (Walnut and others). The slow composting of the lignins is part of the benefit of using used stall bedding as a soil amendment in the landscape. If you are buying it as fertilizer, though, you are being mislead.
Composted stall bedding is not fertilizer. The N from manure and urine does get used up in breaking down the bedding. It makes a very good soil amendment when (garden) soils are low in organic matter.
I have used almost fresh stall cleanings in vegetable boxes and even though I could smell the manure and ammonia the seeding plants did not get burned. It is not that rich.
Horses that are in runs without bedding drop their manure wherever they want (including in the water) and may or may not urinate in the manure.
If you put this material (essentially pure manure) in a compost pile it will probably compost just fine by itself (C:N ratio of horse manure varies with the horse's diet, but is usually about 25 to 35 parts C to 1 part N, great ratio for composting), but the finished product is not fertilizer.
If you steep fresh or nearly fresh (not composted) manure in water to make manure tea there is enough N, P & K that the resulting tea would act as fertilizer in an aquarium, or for house plants. There will highly likely be other materials that would make me think twice about using it though. Domestic horses are regularly wormed, and some of this material ends up in the manure. Horses are fed salt (usually sodium chloride blended with minerals) and some of this ends up in the manure. Is this too salty for aquarium fertilizer? It might be. In garden or house plant use the manure tea is not the only source of water. You would use manure tea occasionally, and water several times, flushing out the salts the plants do not use and do not like.