I don't think an acid stop bath will help reduce the streaking but it will make your fixer last a lot longer, assuming it is an acid fixer. Use water if it is an alkaline fixer.
Your local car spares shop should have distilled/deionized water by the 5 litre bottle for a few Euro - it is used to top-up batteries. Use that in your final wash with a few drops of the Agepon. You are correct: too much is worse than too little in this case.
Good luck! Bob.