Spontaneous deseises

Question
Could one of your fish be picking on the others?
         No, they all get along and have done for a long time.

How long has the tank been set up?
         Erm... about 7 months

How often do you make water changes?
         Once a week, 20%

How often is the filter changed?
         Once a month, carbon filter

Have you altered the pH?
         Not perpousfully (sp?), but it has gone down sharply and so my mum and I added a package of Proper pH plus
         
What kind of food do you feed?
         TetraMin Tropical Flakes

I looked at the link you gave, but it seems to have the symptoms more of the fin rot. Fraying. The Platy is now spending the majority of her time BEHIND the heater, though the temperature  is 79▫
The Snow tetra has a mild fray on his dorsal fin, though the white was gone today and the danio's have small puffs on their LONG fins.

I cleaned out the tank too, 25%, filter change, gravel vacuum.  

Answer
Hi Naomi;

There may have been an ammonia spike and the fish now have infections in the wounds caused by ammonia burns. Livebearing fish like platies are especially sensitive. Or, if there is overfeeding and too few water changes the fish have an infection related to water quality. The chronically low pH is a good indication of that.

Make an extra water change of 25%. It will strengthen the fish and help them heal. Twice a week for the next two weeks would be good.

Avoid altering the pH with chemicals and additives. It causes fluctuations that are more stressful to the fish than a seemingly 'bad' pH. Frequent water changes bring the situation under control safely and slowly. Gravel vacuuming every two weeks to a month reduces excess waste that is often responsible for low pH. Cut back on feeding if you have dirty gravel or think it may be a problem. Avoid changing the filter at the same time as gravel vacuuming. It disturbs the bacteria balance too much and is one of the causes of ammonia spikes.

Your fish may need medication, but look at the pictures and descriptions on the packages at the store to see what you need. Bacterial infections can look like fungus, so read carefully. Here is a great page on our own About.com site to help you with that as well;
http://freshaquarium.about.com/cs/disease/p/finrot.htm

Hope they feel better soon.....

Chris Robbins