If you watch the video through you will see these strange white rock crystals at the end. They made these. If your pH gets too high in your system these rocks magically appear in your water column and start to snow down onto the bottom. That's where your NUTRIENTS go when the pH of the solution is out of their range for the solubility of those chemical species.

I think... I posted an article or something on how to make these out of eggshells. I made some giant ones years ago and used the rock pieces to feed nutrients to my plants. You have to extract the calcium from the eggshells (although some people have thrown eggs into their system as this works, but very differently and it is less predictable given the rate of chemical reaction). It's fun, easy, and maybe a really cool science project to do with the kids (if there are kids that is).

I hope something here helped!!