First, locate where the leak is coming from. Make sure you mark the area with something (maybe a marker) so you can observe it, next drain the tank immediately and hopefully it's something simple and you can just apply some extra silicone in the area. Definitely go heavy with the silicone and cover a larger area than where the leak is coming from. Hope this helps. 300 gallons is a huge tank!
I would like to see your leak area. It could be a simple silicon fix as stated or possible glass removal and reinstall after opening is repaired and prepped to put glass back in.
The product is pond shield. It is an aquarium safe epoxy paint. I purchased it from Amazon. Also, the corners get fiberglass mesh. It's a pain to work with, but it is a requirement. Good luck with your build. 👍
