Hello Sumita,
It is internal infection that cause to this.
Since he is active so no worries.
I suggest you following treatment for your flowerhorn –
Below are treatment process :

Medicine Required :

1) Grand Fish clear Aqua safe – Buy from here
2) Grand Fish Anchor clear – Buy from here
3) EPSOM Salt

Treatment Steps:

1) remove decoratives

We are first cleaning tank –

2) remove Flowerhorn and keep in Bucket of water

3) remove all water from tank

4) clean all surface of tank

5) put some warm water in tank n wash tank

6) clean heater, filter and bubble stone

7) now fill the fresh water- Not bore well or lake water.

8) add half Grand Fish Clear Aqua safe

9) add half cup Grand Fish Anchor

10) install heater and put temperature at 30° (on it)

11) Install filter and air stone(oxygen)

12) after 20 mins add fish to tank

13) every week 20% water change and half cup AntiChlorine n half cup Gill, fungus parasites special

14) in two days we have to give him ESPON Salt treatment

The EPSOM treatment are below –

1) take a bucket and fill 20 ltr fresh water

2) add four spoon EPSOM Salt to Bucket

3) now keep Flowerhorn to Bucket for 15 mins.

4) remove immediately if you see Flowerhorn is uneasy

This is process we need to follow.

Feel free to write back to me for any doubts or questions.

You can also buy medicine from my website too.
I am also mailing you the my number for further assistance.
 
Thank you,
Yashwant Naik