I bought 4 parks combo tickets and went zoo and river safari the same day. These 2 parks are very near and just next to each other.
Expected to see diff tanks and fish, which in fact the tank is well decorated compared to other theme parks so is a pleasant to see. Take the boat to see the lake but expecting it is quite hot as there is no fan not air conditioning.
Considering usually staying Singapore just few days while there are lots other things to do, I will have it on to-do list when there are spare time