It could be a tumour or the start of an ulcer. Monitor and report any changes.
The fish also looks pretty skinny in its back. Make sure you feed it a varied diet including lots of plant matter and prawn. If it continues to remain skinny perhaps deworm them.
Praziquantel will treat tapeworm and gill flukes.
Levamisole will treat thread/ round worms.
You normally treat once a week for 3 weeks or as directed on packaging. Do a 75% water change 24-48 hours after treatment.
Do 75% water daily water changes to get the ammonia down.
How long has the tank been running for?
When was the filter cleaned?
How did you clean the filter?
