I cook mine in the microwave, steaming them basically, covered with plastic food wrap in a container with a little bit of water.
THis method is recommended to people cause the vitamins stay in best, a lot of water makes all the vitamins get thown out with the water, I'm assuming it's the same with fish.
Cooking them makes the vege soft so they can eat them, plus they will sink that way, if you've got plecos (I do)they love it. But all my other fish have their nibbles as well. If you've got peas, cook them then squeese out the inside and crush it up with a fork, throwing out the skin.
Apparently you can wilt lettuce. I use brocoli, peas and silverbeet for my fish.
They may not eat very much (other fish) whereas my plecos will devour a big handfull of vege overnight. Plus ALL my plants
If they don't get eaten within a day, take them out because the vege goes slimy and rotten and polutes your water.
My fish prefer the brocoli over everything else I think