completed my second pond last year and yes if you render it on block use either a decent plastering sand with water proofer and fibre entrained ,then when completely dry get it fibre glassed by some one who knows what they are doing, or fix kingspan to the walls and floor and glass those, keep you water warmer in winter months or save money if you intend heating your pond (pointless spending all that money on reinforced concrete bases and blocks to scrimp on the finish and your fibre glass will way outlast any pond sealer) Are you fitting a bottom drain ,what filter ,returns ,skimmer, gallonage ,pump etc are you planning to use , take my advise and plan it well and do it right first time because every koi keeper in country have built ponds and thought " i wished i had done it this way" plenty of websites and forums if you need any expert help and there is a national BKKs show on this weekend near Nottingham i think always plenty of stuff on offer and expert advice