Can excess food poison your tank

if I feed my goldfish and guppy somtimes the food goes down to the bottom can that leftover food poison my tank

Yeah algae and other organisms can start growing on the nutrients as they dissolve in the water. Clean out what you can and get a good filter for the rest. A good filter has a material like gravel with a large surface area for micro-organisms to live on and consume the nutrients. You wash off extra organisms (slime) from time to time to keep it from clogging. You can also use aquatic plants to absorb nutrients. They're probably even better at it.

A clever trick a friend of mine used to vacuum gunk was to cut the top off a water bottle to make a funnel-like thing. Then he shoved it into a plastic tube. The plastic tube drains water out of the tank right out his window. He uses this to vacuum gunk off the bottom of his tank without sucking up the heavier gravel. He drains 1/4 of the tank water at the same time, and refills it afterwards. You're supposed to drain out some water every couple weeks anyway.