Do you need GFO: how to use it...RE PHOSPHATE AND ALGAE PLAGUE
Granulated Ferrous Oxide is, in effect, rusty iron pellets made porous, specifically to sop up and hold phosphate out of your water.

Rock, sand, and tapwater all contribute phosphate to your tank, and algae loves it...especially hair and film algae. Some rock is worse than others in this regard---but it can be some of the prettier rock. And 'new' rock goes through a period when it starts slowly 'leaking' phospate...

Buying a marine aquarium
...most fish stores cannot stock the size of tank appropriate for most of the fish they sell. Tanks of 30 gallons or under are appropriate for a clown pair, a few gobies or blennies, a royal gramma or dartfish, and some inverts. And with good lighting---corals.

If you want larger fish, you have several size jumps. A 50 will let you have a dwarf angel or similar, anything a 30 can handle, maybe a single chromis (a damsel, like a clown, mildmannered,...

Understand, you can run a very simple kind of 30 gal tank with nothing but a pump, a heater, live rock, regular tank light, and a Penguin filter, with a 'fill line' drawn on the glass to help you top off accurately with ro water from the supermarket. This is the ancient method. It will support a few crustaceans and snails and a couple of very small gobies or blennies or dartfish.

If you want more than that, your problem becomes too much nutrient in the system, not enough waste removal,...

I've read an increasing fervor for dumping various things into a new tank to cycle it, many with the implication it does it faster---and some involving a lot of extra expense.

Cycling is a simple natural process of letting bacteria multiply and penetrate more than just the outer skin of your rock and sand. And bacteria multiply quite nicely without your doing more than setting up with one live rock amid conditioned (saltwater soaked) plain rock. I've even done it with no more than...

Sand: buying it, keeping it.
Avoid silicate sand. Go for aragonite, which is basically calcium carbonate and safe.
Fine or medium? I can only tell you personally, with an sps reef, in a re-set-up, I tried fine, and spent the next 2 years removing it, after it killed corals. I'm definitely on the medium side. The fine sand, though amply bacteria'ed, would not settle, blew like mad, irritated corals, and was a mess. The medium sand has enough 'shape' to it to enable jawfish to build...