The water zoo does cater for all from pond to Marine and stocks the more rare species which the barn garden centre, p@h and maidenhead aquatics don't get at all, they have some really nice corals and equally stunning fish. Pity about it's location as Lincoln road isn't the best of roads to be on (no offence meant but 90%+ made up of immigrants all of which don't get on too well). Either way will be ending up there at somepoint tomorrow.
As for plants I recommend the barn garden centre near Paston as they actively ensure to the best of their knowledge that they will be snail free as you can't buy snail control chemicals in EU due to possibilities of leaking to rivers etc.