We will discuss the bait later, first lets go through the rig you are using. As for as I know, you must be using bolt rig for targeting the Rohu. (I have fished with some anglers from your part of wood and the way they where fishing was bolt rig so I am assuming that you must be using the same rig.)
I think, the bait is not the only area of concern, while targeting a particular fish, hook, line, setup, method and presentation all play a very important role.
Rohu feeds by sieving the water through its gills and the best way to target them is by presenting the bait on or just below the surface. Before going for fancy baits, I would prefer using a piece of bread or some local weed (Including Alga) to target it.
If the fishing pressure is not very high, I am sure you will be able to catch this fish very easily by adopting the above suggestions.
You can also target this fish by stalking the fringe and using a zero balance setup in early morning or dusk time.
Any bengali friend will vouch for ant eggs but I am not sure about that. Any bait, which is creating a cloud of particles around it, can be used. If you have added any sweetener in your bait, the same can be used for Catla also.