How to get a fat stack of Bio Toxin :

1. Get some good armor / level up your health (so you dont die if you get electrocuted)

2. Get a Stone Hatchet or a Metal Hatchet

3. Find some Cnidarias

4. Get into the water, and lure them out

5. Get into land, and just start chopping them with your hatchet

6. Repeat until you get the desired amount of Bio Toxin.

Good luck.