Tom,
I can't answer the question about the biocide, but I can offer you a suggestion for a contraption I use to safely and accurately dispense additives to my fuel tank. Being the cheap tightwad I am, this contraption costs very little, if anything.
There are two optiions:
If you have ever bought differential oil in a quart, chances are it came with a pointed spout.
If you or someone you know uses Clairol hair dye products, you know one of the ingredients is dispensed from a plastic bottle with a pointed spout.
So using one of these bottles with a pointed spout, if you get some flexible plastic tubing to fit over the pointed spout, you can prefill tthe bottle with the desired amount of additive and safely dispense the liquid into the target.
I use one of these to fill the deep cycle batteries in the coach with distilled water.....obviously not the same dispenser as I use for fuel additive.
Hope this helps!