If you're mixing the water with Prime separately, add only as much as is needed for that volume of water. Personally, if your water has ammonium or ammonia in it already, I'd do this so as not to flood your fish with ammonia before the Prime takes care of it.
If you put the Prime directly into the tank and add water to the tank, use as much as would be needed for the entire aquarium's worth of water.
If you have a ton of nitrites, just do a water change, don't fiddle around with adding a ton of anything.
Sponge filter gunk will definitely give you a boost, it's just hard to tell how much of one.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
My current project, a 65 gallon aquarium stocked with vernal pool fauna.