One other question, since I'm now using RO/DI water, and RO Right, the RO
Right does not register on my GH test. The (TINY) instructions on the
packaging say that typically GH tests measure Ca and apparently there isn't
much in RO Right.

How important is Ca in an aquarium, both to the fish and to the plants?

Someone suggested "pool water hardener" but I'm concerned that 1. Would I
really know what's in it, 2. Anything that is for a pool, must be very
highly concentrated.

My thought is crushed coral in the filter, is this a good idea, and is there
anyone out there near the ocean who'd help me out with some via postal
service? I'm willing to pay the shipping costs plus $3-4 bucks for your
time. I assume (?) that just maybe an ounce of seashells that I could crush
would do the trick?

I'm posting this here instead of marketplace because I need to know if this
is the right idea.

thanks,

bob