The longer you leave it the more chance of it not getting sorted aswell dude, just keep on their case everyday and make sure they know you are completely disgusted with the quality of the workmanship and want a full refund. Don't accept anything less!
If I still lived in chch I would have loved to help you out dealing with them as I think from what you have told us that they are just taking advantage of you which is wrong.
When someone supplies a goods or service that is not up to scratch or not be able to be used for the purpose for which it was intended you can mention the consumers guarantee act, I have done this many times and it usually makes people/businesses take notice and do the right thing for the customer.
The only problem you have is that you have lost the receipt, this is not a serious issue though as because you got the tank made from a reputable business I don't think they will deny making it without some serious trouble, just focus on the quality of the workmanship and not so much the wrong size and you should be able to get a refund.
Last but not least I am wondering if they offered any sort of guarantee on the tank?