Mystery smell

We've been having the weirdest problem with out new fish tank. So after we set it up on New Year's eve, and put the fish in, we went about our business as usual. A couple of days later we came home from running errands and Oh-My-God the house STUNK like cat pee. So of course we accuse the cats and start sniffing around, but no... it wasn't the cats. It was THE FISH! I am serious when I say that the water in the fish tank smelled like cat pee so bad that it burned our eyes. So I took a sample of the water to Petsmart, where they tested it and said there was technically nothing wrong with the water, except the nitrate levels were a little elevated, but they did smell it. In fact when she took the lid off the water bottle she almost fainted.

.... so we set up their old tank, put them back in and I cleaned out the new tank and all the accessories with bleach water, per their instructions at Petsmart. After thoroughly rinsing out all the bleach water we set it back up, put water in it, ran the filter for a day, and returned the fish to their new tank.

Oh you think it's all over? no. 2 dyas later, deja vu. The smell is BACK! WTF? So we did a partial water change but it didn't really help.

I called an aquarium shop and the guy said it might be plastic eating bacteria from the accessories sitting in a closet for years, and to get rid of all the plastic plants, ornaments, etc.

So again, we set up the old aquarium, and we transfer the fish. And I dispose of everything from the new tank, except the gravel, which I rinsed thoroughly with hot water. We then put the gravel back in, and all the water, and are now letting the filter run for a few days. However, this morning, there was a faint smell again. I'm going to see if it gets worse over the next few days. If the smell does come back, the only conclusion I can draw at this point is that the bag of gravel we got from Petsmart is causing it!

Poor William and Harry. This morning when I went to feed them they were completely uninterested and just cuddled up together at the bottom of the tank. They have no plants and no big rock house to hide in and I'm sure they're completely freaked out. If the smell is back tomorrow, I guess I'll have to buy new gravel and set it all back up again!... that is, if William and Harry don't die first. :-(

I'm frustrated, and I'm afraid that my boys might not make it because I wanted to give them a bigger house, which will cause me enormous guilt. Even though they're just fish. I can't stand to think that I killed them.