Raw vegetables need to be left in the water for a bit longer, especially when first introduced. It takes a while for fish to recognize veggies as food, and most veggies need to be clipped or weighted to stay submerged and stable enough for many fish to feed on them. Many people blanche (submerge in boiling water for 30-60 seconds) or freeze the foods, though this is not needed. Not all fish will take to veggies, while some species really need them--what are you offering them to?
Some foods to try: cucumbers, zucchini, leafy lettuce (not iceberg--little to no nutrients), spinach, broccoli, and cauliflower. Peas are readily taken as well, but will act as a laxative. Celery also has little nutrients to offer, so I wouldn't offer it.
I feed fresh veggies to my gouramies, bristlenose, chocolate loricaid, whiptails, cories, mollies, kuhli loaches, and even my ACF. I leave the veggie in the water for 3-4 days, and have never had a problem with it breaking down or polluting the water.
Another quick point--fish graze on veggies, rather than slurping them down like blood worms or flakes.