jw wrote:

I have never understood the purpose of the sand in the smaller tank. It
would seem that if the water were pumped under the sand, as it rose through
it to the discharge line into the larger tank, it would be filtered. How can
the sand be expected to filter anything as the water simply flows over it?

The general idea is that nitrifying bacteria grows in the sand. The sand doesn't
serve as a filter in the sense that you mean it. The sand can also serve as a
basis for supporting filter plants such as mangroves.

How quickly should the trace minerals be put into the tank after a partial
water change?

Assuming you used a good marine salt mix (such as Red Sea, Instant Ocean, or
Tropic-Marin), the new water will contain the correct amount of trace elements.
Just stay on your usual schedule.

Also, does the recalculating charcoal filter in the
smaller tank rob the minerals that I put in to feed the coral? With a total
of 45 gallons how much water should be exchanged when changing the water?

The filter should not do that. Usually people change about 1/4 of the water at a
time. Given that one common size for salt mixes is ten gallons, that's what I
would change in your case. Opinions vary as to how often you should change
water. Since my tank setup is different, I will not offer an opinion.

As for the algae, it sounds like you have hair algae. This usually feeds on
nitrates and/or phosphates. Nitrates are an end product of food decay. These
should be low or non-existent in a properly set up reef tank. Phosphates tend to
come in from outside, usually from the tap water. There are two things you can
do about this, and you probably should do both.

If you can find a source of clean water produced by running it through a
reverse-osmosis filter, use that for your water changes. Distilled water is
another option. A third option is to use bottled spring water; if you do this,
make sure the brand you buy contains no phosphates (fish stores sell test kits
for this).

The second thing to do is buy a few blue-leg hermit crabs. They eat algae.

Finally; make sure that protein skimmer is working correctly. This is very
important with your setup.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.