I am assuming you did all the due diligence and had the necessary land investigations to determine that fishery is viable on the site, id hate to think youll spend a lot of money on developing a pool/lake that disappears into the water table :P
On a more serious note planning can be a nightmare, i dont know wher you live but thers a weekly (i think) publication called Midlands Angler that is currently or recently serialised articles on someone who is completing the process you have just started, that might be a useful read if you can get hold of it.
fisheries4sale.com is run by estate agents/surveyors who specialise in this and will be able to provide advice on planning, build etc. i dont know what scale you are thinking of but its worth using a consultant like this as planning permission is notoriously fraught with tripwires and doing it yourself could end up with a rejection on a technicality or simple error. their details are:
Fenn Wright: Chartered Surveyors, Estate Agents and Property Consultants Chartered Surveyors, Estate Agents, Property Consultants, 1 Tollgate East, Stanway, Colchester. CO3 8RS
good luck, keep us all posted on your progress, that will make one hell of a blog!