my mollys have died but the rest are fine.

Question
My tank has been set up for about 10 months, it is a 12 gallon tank.My ph is at 6.0 and my ammonia is at .5  I started my tank with 4 mollies and 1 very small frog and a small catfish.  The mollies were fine for a long time, even had babies and one survived and got as big as the other mollies. Oh,I had snails from the plants all over my tank too. Then the mollies one by one died. They would be dead on the bottom of the tank.Each one seemed fine the day before. And ALL of the snails died too. But my catfishand frog are fine. It has been about a month since the last molly died and I am afraid to add more fish. What is wrong?

Answer
Hi Anita;

I would suspect that the pH has dropped too low, even below 6.0 from time to time and the beneficial bacteria in your tank and in the filter has not been thriving like it should. It can't do very well at pH under 6.0 and may start to die off. That's why the ammonia is up. The beneficial bacteria should be controlling the ammonia but it can't if it's dying off. In an established tank the ammonia should always be "zero". This has probably developed slowly over time and has just gotten to the point that the mollies couldn't tolerate the water chemistry anymore. Mollies require very clean water. Frogs and little cory cats are much more tolerant of adverse water chemistry.

You need to make partial water changes of 25% once a week in your tank, and vacuum the gravel every 2 to 3 weeks. Your fish need the electrolytes and minerals that fresh new water provides. Over time, these trace elements are "used up" by the biological system that is your aquarium. They must be replenished on a regular basis in order for that little closed environment to stay healthy and balanced.

To get things back to normal as soon as possible without shocking the current fish, make a 25% water change every other day for a week or two. Vacuum the gravel every 4 or 5 days with those 25% changes. Crud in the gravel will cause low pH and chronic ammonia spikes too. Once the ammonia is gone and stays gone, you can add one or two mollies again. Go slow and add only a couple at the most and then maybe another two in another week. Your tank can't handle more fish than that, especially once they start having babies.  Mollies get about 2 or 3 inches long and the little catfish and frog probably about an inch each or a little less. Your tank can hold a total of 12 inches of fish. You go by the adult sizes when you calculate fish population.

I hope all goes well!

At Your Service;
Chris Robbins