dechlorinator is something you add to the water when first filling the tank and everytime you replace water in the tank.
dechlorinator ranges from £2 a bottle to £11 for a huge bottle at my LFS, this is essential for the fish to survive,, this product removea chlorine from the water which if left in the water will harm your fish and probably kill them.
Your pump should be on 24/7
I prefer gravel. Some fish require a sand substrate but none of mine do. Substrate choice depends on what fish you want to keep,