My Congo was kept in gravel before and he was fine and he moved a lot more around rather than digging in the gravel. However, they will prefer sand as digging is one of their things and its really interesting and funny to see them making their home. Mine is kept in a medium current where there is enough current for him and is still not too powerful for my other cichlids and loaches.
I know this might not help because I haven't gave a real answer lol but they really do prefer sandy substrate so yeah its best to change if you really want to keep them. They like strong currents but its not that they cannot swim up or fast without it, if you provide them with a medium current so not too strong or too weak then that will be good, then its not stressing your other fish.
If you change to sand it will benefit in different ways and most fish prefer a finer substrate. Also one other piece of advice, if you do get sand then go for a darker colour as your fish will feel more secure. You will see your Congo move around a lot more, thats why I am changing to a darker sand substrate.
Hope this helps
I know this might not help because I haven't gave a real answer lol but they really do prefer sandy substrate so yeah its best to change if you really want to keep them. They like strong currents but its not that they cannot swim up or fast without it, if you provide them with a medium current so not too strong or too weak then that will be good, then its not stressing your other fish.
If you change to sand it will benefit in different ways and most fish prefer a finer substrate. Also one other piece of advice, if you do get sand then go for a darker colour as your fish will feel more secure. You will see your Congo move around a lot more, thats why I am changing to a darker sand substrate.
Hope this helps

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