Blue Gourami Three Spot With A Red Spot

[Laura.p]

I've had my threes spots for about 2 years now. I've got two of them.
They are very lively.

One of them has a dark red spot on one side of the body a couple of mm from the dorsal fin.
It looks dark blood red.

What is it and what should I do to treat it?

 

[Laura.p]

The tank size is 180 litres.
The fish I have are the 2 blue gouramis,
2 yellow leopard tail guppys.
2 cory catfish
2 glow light tetras
1 pleco
5 neon tetras. *new additions 6 days ago*

I change about 15-20% of water every Sunday.
I put de-chlorinator in when I change the water.

Should I buy melafix?