Why is my spinach (but nohing else) turnng yellow?

[badflash]

I started with about 1/4 teaspoon per plant. I waited to see how it effected my water quality. Then repeated once a week. The plants greened up. If you over do it, the water starts to smell like a sewer and the fish will be gulping for air.

I use blood meal, but only a little at a time.

Badflash, what do you mean by "sparingly". Can you be a bit more specific? Thanks much.