Multiple deaths in community aquarium

Question
Hi
I bought some swordtails, mollies and a algae eater atthe weekend to go in my new tank.  The tank had been set up empty for 6 week to stabilise.  Less than 48 hours later they had all died.  They seemed to have white patches on the edge and centre of their fins.  My PH is 7.4, ammonia and nitrite levels are 0, nitrate is 5.0.  The tank is a 54 litre.  My phosphate level was high but could this have killed them.  Also could it be something fundamentally wrong with the tank itself as I have kept coldwater fish in it in the past.  Please advise.
Thank you,  Kath

Answer
Hi Kathryn:  What is the tank temperature?  Stress is usually the number one onset of disease in fish and coming from the fish store to your home is very stressful for fish.  The process starts with how they are captured in the store... I have seen clerks chase the fish around with the net trying to catch them... the longer the chase the more stressed the fish is... add to that the bagging process, the ride home and then being placed into a tank that is new and potential full of dangers and the fish ends up sick.  

The nitrate in your tank is high but nitrate is the mid point in the ammonia breakdown cycle I always recommend at least two live plants in the tank... plants use nitrate to feed on, etc. But the real issue is what is causing the ammonia...especially in an empty tank... the answer is probably left over fish food and fish feces in the gravel.  I would do a number of things to correct this problem.

1. Clean the gravel with a gravel vacuum while changing about 20% of the water and take out all of the decorations and clean under them.

2. Add two plants to your tank... your tank is small about 14 US gallons so one or two plants should do you well.  

3.  Make sure you have plenty of oxygen in your tank water... add an air pump and an airstone (bubbler).  sometimes high nitrate levels will lead to nitrate poisoning but oxygen in the water will help to lessen the affect of the nitrate.

4. When you add your new water to the tank treat the tank with aquarium salt... and each time you do a partial water change you will add more salt for the new water only.  

I know losing fish is a horrid experience... but you have not done anything that is grossly wrong... A few corrections and I think you will be ready to try again...  I hope this helps and if you have any further questions please let me know... dave