I loaded them for a specific reason which was SBFT. They seem to have a particular liking for the crystal minnows. I wanted to be able to cast them into a wind at a good distance. I filled the minnow completely with tap water. I don’t think growth in the water would be a problem. The plug is plastic anyway. In order to load it I heated a paperclip and poked 2 holes in the minnow. One right behind the eyes and one about 2/3 the way down the body from the head. I used an epoxy syringe with a plastic needle from the marine store and put the tip in the tail hole. I filled the bait until water came out the top hole. The two holes were necessary because I was using a rather large plastic syringe that didn’t fit through the hole. If you can find a small metal syringe the second hole or carburetor wouldn’t be necessary. In order to close the holes I heated a knife tip and spread the plastic over the hole.
I use a really fast retrieve while tuna fishing the so extra weight did not affect the action very much. In fact gave it a little more control with the quicker retrieve. It lost some action on slower retrieves but I’m sure if you played around a little you could find a good balance.
This is the same procedure I use to weight 7”
redfins except I use a bucket full of water to tell me when there is enough water in them. They float at about 45 degrees in fresh water with 3/0 VMC’s and splitrings. Dennis Zambrotta wrote a good article on this.