Is it okay if I keep my flowerhorn in a 15 gallon tank? Is a tank which is Length = 60 cm Breadth = 30 cm Height = 30 cm 15 Gallons? 
Okay bro, once it grow big then I'll consider a new tankMaybe as a grow out? Needs, absolute minimum, 55g, 75g better.
is 60 cm x 30 cm x 30 cm = 15 gal ?15 gallons really isn't a lot for a grow out, a 20Long or a 40Breeder would be better. The fish's growth might actually be stunted from being in a 15 too long, and other health issues and stress will be a factor. I currently have my 6" FH in a 55 and i'm looking to do a big upgrade in tank size so that he can max out in size and have plenty of room to move around.
Okay dude once my FH hits 3.5 inches i'll buy a new tank. Thanks for your adviceYour tank is about 15 gallions. I think the 15 gallon would be okay until he or she hits maybe 3.5 inches MAX. Then maybe move it to a 40 breeder a 55 gallon but a 75 would be Ideal. The faster you move your FH to a bigger tank, the better.