need a do-over

Question
I have had a 12 gallon tank for about 6 weeks.  We started with two small mollies.  One died within a few days, and we added two more, one of which turned out to be pregnant.  The pregnant fish spawned, and the other fish and/or the filter ate all but two of the fry.  The ammonia spiked!  The mother fish died, soon after one of the fry died, followed about two weeks later by one of the other adults.  So we were down to one adult (the first we had bought who we figured must be a hearty soul) and the remaining fry.  Another week, we lost the adult and just yesterday the baby died.  I tried to follow directions, change 1/4 of the water every couple of days, etc.  We now have zero ammonia but still high nitrates and our nitrates are at 40.  Ph has been pretty steady at around 6.8 I think. My kids of course want to repopulate the tank as soon as the nitrites are down but I have concerns about our ability to keep them alive.  Is this just new tank syndrome?  Were we foolish to start with Mollies?   What would be a good population of easy, hardy attractive fish for this tank (which, by the way, is an Eclipse 12 system).  And how long can the tank be empty before we the good bacteria start to die off? Thanks for any advice.

Answer
Hi Judy;

It was all from new tank syndrome for sure. There were just too many fish to get it going safely and toxins rose too high. You can add more fish once the ammonia and nitrIte are "zero" and have been for a week. The beneficial bacteria will be just fine. I'm sure there's still plenty of waste for them to continue to feed on for at least a week. Don't make any water changes though. There are no fish left for toxins to harm.

About nitrAte;
NitrAte is the end result of your tank's biological process. As long as it stays below 40 ppm, 20 ppm or lower is even better, it's fine. It continually rises over time and is removed by our partial water changes. There isn't any process except feeding on it by plants (and it takes a lot of them) that breaks down nitrate further in an aquarium. It's just not a large enough eco-system like a lake or a river would be. If you find that any toxins stay high even after water changes, test the tap water right out of the faucet. Sometimes it comes equipped with it's own supply of toxins before you even get started. It's a good thing to check out just to be sure.

Once you start adding new fish, go VERY slow. It would be best not to get mollies again though. They actually need a larger tank of at least 20 gallons. Especially once they start having babies. Mollies grow to be over 3" long as adults and do make quite a bit of waste for their size. Mollies actually do better in a higher pH with harder water anyway. Platies stay smaller, but again, they will have babies that will eventually overpopulate your little tank. Or, just get a couple of male platies. Provide plenty of hiding places for them so they don't get so territorial. Have you considered white clouds, danios, neons, cory cats, cherry barbs, or t-bone rasboras? They all stay on the small side and like to be in groups of 3 or more. They aren't very messy either. You could get 3 of one kind to get things going again, watch the levels and then add 3 more of the same (or another kind) the next week. Adding a small number of fish and waiting a week will let the nitrifying bacteria compensate.

At Your Service;
Chris Robbins