Okay, small diameter acrylic tube with a plastic brush attached to the end and attach a vinyl hose to it. Syphon it out as you brush it. I've done it long ago and it worked. Hey, that's the best I could come up with! Lol
I have dosed Kno3 and still have some in solution in a gallon jug. Always had 0 No3 and Po4 and was feeding heavy. Took several days of dosing to get it up to detectable. Now I keep it at 10ppm and po4 at trace and hardly ever dose it now.
"maybe buying 3 pounds of the stuff was overboard" Yes...
If it's a temp tank, just silicone a piece of glass over the hole and drill a new hole in a different location. If it's to be more of a permanent tank install, drilling a larger hole around the existing hole could be done. Carefully.
Got the same stuff on one of my overflows because of sun light coming into our living room in the afternoon when the blinds are up. My tomini loves it.
I second taking the rock out and hydrogen peroxide solution the offending area. Should work.
Depending on the size of wrasse, small fairies especially, they may get through the egg crate. Build a screen top. Everything you need to make one is at HD or lowes. Cheep and will let more light into the tank than egg crate.
My wife can be a firecracker at times, but she understands that the tank is my hobby. She's helped me numerous times with it when I needed someone's help holding something, lifting etc...... but it's still "my thing". Attempts to include her over the years in it leads to her just not wanting to...