installing!!!

First check your water conditions. It shouldnt have any chlorine content. Sharks get very big and produce a lot of waste, which will shoot up the waste level in a small tank quickly. so its good to avoid such fish. do a water change every week and give the fish good nutritious food. Dont overstock the tank.

I do think your have a small tank for a shark, mollies and guppies. You provided an incomplete measurement of your thank so I can not determine how many gallons/liters it can contain. Anyway, try to decrease the number of fish you have or if you want all those fishes, get a bigger aquarium. Properly installing means you have the necessary equipments (e.g. aerator) functioning properly in the right places. You also have to make sure that you are using the right water quality without any pollutants (chlorine, soap) in it. As I have mentioned earlier, you should also have the right size of aquarium and the right fish combination (you should not put a pair of fighting fish in the same tank).