I give our goldfish lettuce, spinach, cucumber, and peas. Once or twice a week, and just a bit - like 1 or 2 peas, or 1/4 lettuce leaf, 1 spinach leaf, or 1/4 of a slice of cucumber. Steadily, just not a lot at once. I don't give them broccoli (too stinky and they don't touch it) or starchy vegetables.
Blanching is important, and it is easy. Some here don't do it and are not a fan of it, but I'm sticking to my training from an old aquarium expert: boil the veg for one minute and let it cool. This breaks down the fibrous outer layer and allows easier access to the food. You may not do it with lettuce (a few people here have reported they do just fine without blanching lettuce) but peas definitely need a little prep to be edible. This means squeezing it so the inner soft part pops out of the outer husk, after blanching. I'm talking about the round outer layer of the pea itself, not the big long pea pod that houses several peas. Creatures can't access a pea unless it is popped out of that outer layer.
Most veggies will float, which could appeal to some fish. I skewer them on a chopstick, pushing the food about halfway up the stick so it stays put for them to nibble. If I want snails to eat it, skewering the veg so it stays at the bottom of the tank will do it.
I also give them dried bloodworms and dried crustacean food ( small containers found at aquarium stores) on occasion. They love both.