Hi Speedy.I would have serious reservations about the tank.Unless it is made of plastic it is not good for water to come into contact with anything else.This is a firm belief i have held for many years,being extremely cautious about what i put into my fish and nature ponds,and the theory seems to hold up.
Secondly,a nature pond really must have a gentle slope out of it for the amphibians.Frogs and newts can certainly jump or slide in but getting out again,which they must be able to do,is just not possible from something like a steep sided tank.
Rigid plastic ponds can be a compromise if you can find something to place onto the provided shelves that the frogs can climb or jump onto before getting out but the only really suitable way is a liner pond.It should have a shallow area leading gently out,and a deeper part for the frogs and newts to overwinter in the mud.It can be made to look far more natural around the edges than any prefabricated pond and indeed the more plants you put in around the edges and behind the pond,the better for the shelter they need and the insects that are attracted there for food.
Which just leaves the problem of the weeds penetrating the liner.Easily solved,this one.Dig your hole,at least 18 ins deep or more if you can.Obviously clear away as many of the weeds as possible.Line it with a good layer of sand.Then place on top,a nice thick piece of old carpet.Make sure it comes up the sides as far as possible.Then place your liner over that.Use a good quality one,which again will help prevent problems.Place a good layer of garden soil in to the bottom,but make sure it has not had fertiliser added.Then run in the water at a gentle pace.
It will look awful for a few days but will settle,then you can start planting up the plants,oxygenating weed etc.You cannot overplant a nature pond.The more you put in,the better chance of keeping the water free from algae.Beg a bucket or two from another pondie to get some insect life going.then stand back,and wait!! It shouldn't be long before you are rewarded with slug control!!
Don't be tempted to put a cover or grid over as this may prevent any hedgehogs getting out.If there were to be a risk from small children then a fence around it would be better.