i have a bit of bga in the substrate, between the glass, any ideas on how to treat that...?
Take either the amount of H2O2 or Metricide you're already using. Add water to it to increase the volume - the reason, and amount of water, will become evident shortly.
Turn off filters. Suck up your H2O2/Metricide solution into a syringe with a needle. Inject it right into the substrate in the middle of the BGA patch, up against the glass.
The substrate tends to hold the chemical right up against the BGA, so it won't dissipate as fast as in a normal spot treatment. As long as you have enough volume to fill this area of the substrate, even a diluted solution will have a powerful effect due to this prolonged contact.
Wait 15 minutes and turn the filter back on. Done.
If the BGA extends up the glass above the substrate line, you can also "tent" that area. Use rocks, plastic, or whatever is handy to make a temporary cover. It doesn't have to be perfect. As the treatment seeps out of the substrate, it will be somewhat trapped in this area, ensuring the BGA on the glass gets a good dose too.
In the ocean, there's a seaweed called Caulerpa. It's an invasive species which grows on the ocean floor. Nothing eats it except one species of slug, which does so only grudgingly. And it crowds out all other plants and corals. So once it becomes established in an area, nothing
else remains. It's sometimes eliminated by laying tarps over the ocean floor, then pumping chlorine under the tarps. So I didn't invent "tenting", just borrowed the technique.
You can get creative with it. For example, you might "tent" a plant by putting an inverted plastic bag over it, adding a treatment with a syringe, and then loosely tying off the bottom of the bag. Kind of like a dip, but inside the tank. You still have to limit the amount of chemical, as once you remove the bag, anything that remains is dispersed into the water and becomes a full tank treatment; but holding the chemical in the bag, and the more limited amount of water within it for as long as you like, would make this a powerful treatment. I haven't done it, and can't imagine why anyone would. But who knows, it might inspire someone.