My experience with these fish is that they are not aggressive toward other fish, but can be obnoxious sometimes. It's really surprising that this is happening to you. I would have thought the venustus would be the aggressor! It may have something to do with the types of fish you have in that tank. They are from different parts of the world, and their "body language" might be the issue. I know that this is true at least for mixing cichlids from different continents. You might have to move one or the other from the tank.
The Pakastani loach dose not hurt the fish It just chases them around. Maybe It has something to do with there colors being similar. I also find that it dose not need the companionship that clown loaches do infact it rarely interacts with the other Pakastani loach. This could be that it is much smaller than the other one.
If it is a single fish it may do this as loaches are social beings and like others of their own kind. I have six and they dart all over the tank and constantly pick at each other just like little kids Scientific name is Botia lohachata and also commonly called yoyo loach.
ya mabe the yoyo is a bit lonely but its difficult to find these guys but when I get a chance I plan on getting some more als the yoyo loach dose not seen to be intrested in the clown loaches mabe they are just too differnt even though ther botia
I think they also have a pecking order like clowns do. I think another one added to the tank might help if it won't overstock your tank. I have 4 in a 20 gal. long and some hobbyists think that tank is too small for them. They seem quite content, but I do plan on moving them to a larger tank.