Help With Diagnosis

I've attached a few pictures of one of my platys. It has a patch on it's body and I was hoping someone could help me diagnose what the problem/disease is?

Also the final picture is of my swordtail who has a little white area which has developed on it's mouth. What is this due too?

Are the two problems related?

I've got a 50l tank with a fluval u2 filter. Ammonia and nitrite readings are zero and nitrate is at 20ppm.

Many thanks for any help you can offer.

2 more pictures, 1 of platy and other of swordtail...

It looks like a bacterial infection on the platy's back and the swordtails could have Columnaris (another type of bacterial infection). Make sure the water quality is good, (ie: 0 ammonia, 0 nitrite & low nitrate). Do a 50% water change and gravel clean the tank. Clean the filter if it hasn't been done in the last couple of weeks. Increase the aeration/ surface turbulence and then treat them with a broad-spectrum anti-bacterial medication.

To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres

When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.

Remove carbon from the filter before treating otherwise it will absorb the medication out of the water.