Hmmm, bit of a mystery then.
Not uncommon in fishkeeping, I'm afraid; fish disease and medication is not very well advanced, tbh, except for the commercial fisheries side of things. Constipation tends to make fish more 'floaty' than sinking, but the peas are worth a try, and certainly will be good for all your fish.
A full set of water tests (ammonia, nitrite, nitrate and pH) would really help though.