since I have gotten my 1991 Geo Metro I have only been able to fill 8 gallons out of the 10 that I should be able to fill it up with, is there anything I can do to clean my gas tank?

How are you determining that the tank is empty. If you are going by the light, that would be about right that you have 2 gallons left when light comes on. You would have to run the car out of gas to get 10 gallons into it. This is not a good idea however.

well there is no light indicator I just have the arrow and i drove the car once till the arrow was at the far edge of E.... and I still only got 8.56 gallons out of it

That seems about right because you should have a 10 gallon tank and about 11.2 max.

Sounds fine to me too