I don't remember exactly what the bottom looked like on the outside, I think it's basically flat. The inside bottom has a bumpy texture. I have the whole pond sitting on a 3/8" plywood base. When I first brought my fish inside (I had to because my outdoor pond isn't deep enough into the ground for wintering) I had them in a temporary pond made from a Rubbermaid storage bin that I framed out using pieces from an old crib. I did that to keep it from bowing out. This temporary pond was a fast and last minute decision. I had no problems with that temporary pond and they were in that for 4 years until I made this new preformed one. I should've been more worried with my temporary one! I've got this one fixed up and am keeping a GOOD eye on it. After all the work I've just gone through, I'd hate to think about starting all over again. But I do think I'm going to consider changing this over to a stock tank in the future if they are more reliable. Here is a link that might be useful:

Find a plumber who is proficient in performing isolation tests. Show him where the moisture has appeared and he can isolate the drain line in that area, pretty much down to each given fixture, to locate the leak. It is possible that the leak is caused by a gap in the bowl wax seal. As snoonyb said, if the leak is in the bathroom and passing under the tile, the grout more than likely will show some discoloration. If the leak is in a drain pipe encased in the slab, the slab in that area will need to be broken out by jackhammer, the repair made, then the site will be backfilled with sand and concrete. The builder is just that - a builder. He danged sure isn't a plumber! Yes, YES, all utility lines run in a slab can, and occasionally do, develop leaks. Materials have a limited life, soil moves putting stress on pipes (causing cracks and leaks), people abuse drains with acids, etc. Best of luck. Find a GOOD plumber, be kind to them, and keep their number for future use and referrals..... OT, today is Hug A Plumber Day. LOL

Yes, it can easily be fixed. You need to replace the rubber washers inside. I suspect you will probably have to replace the "O" rings also. Turn the water supply to the spigot off. There should be a valve inside the house, near the spigot to turn it off. Don't turn the main house power supply off because if something goes wrong during the repair, you're now stuck with having to fix it instead of taking your time. Then, open the valve about half way and undo the cap and remove the knob. Then remove the fitting just below the knob and pull the stem out. At the end of the stem is a screw holding the rubbber washer on. You'll also see the "O" rings on the stem. Here's where it gets trickey....from what I see so far, you'll have a heck of a time getting the cap off without chewing up the screw. If you get it to loosen, your next challenge will be loosening the nut fitting. Then move down to the nut holding the stem in. Once inside, you may find the screw holding the washer in may get chewed up as you attempt to remove it. Most washer kits (can purchase anywhere, even WalMart) have replacement screws in them. Put the same size washer in the seat and the "O" rings on and start reassembly. You may find the seat is also chewed up or at least chipped. This negative stuff mentioned isn't to intimidate you, just let you know what you may expeience. It's an easy job and if you have the tools, you can do it. Once you've done one, you'll see how easy it is. Doing it for the first time may seem cumbersome. Normally, it's a 5 minute job if there are no extenuating circumstances. You can also just replace the whole spigot. Unthread it from the pipe using a pipe wrench and the right sized wrench for the spigot, put some teflon tape or pipe dope on the threads and put the new one on. They don't cost that much, around 10 bucks.

OP: If you are still dealing with this issue - please look again at my original post. You need to understand if this is a system wide issue. If it is system wide, meaning that the plumbing is at the end of its life, you need to re-plumb and the only practical way of doing that is through the attic and side walls. Your flat roof will make this much more difficult. The good news is you can do it with PEX and fish some of the lines into place. You can consider pipe lining -- but if you have copper pipe that has reached end of life - I'd question how long of a fix that would be. It would be something to ask about if you go that direction. Copper plumbing has a lifespan of 50-70 years so 40 years is a little short if it was properly installed unless it was thinwall copper. Ground chemistry, water chemistry, even electrical currents, etc can shorten the lifespan. You may be able to see what type of copper -- K, L or M -- was installed if you can find a length of expose pipe with the printing on it. If it's type M - that's the thinnest type and I'd say you are at the end of life.