[Michael Hoaster]

Hey Minorhero and welcome! Glad you like the thread!

I like your idea of sea grasses and seahorses. I have to say though I think it will be a challenge. You said you are new to salt water. It's good to start with less difficult species to learn the skills and have some successes, before trying more difficult creatures.

Let's get into your questions. I never tested PAR, but considering the depths grasses inhabit, you'll need lighting on the brighter side - like lighting that would be considered bright reef lighting. If I were to test PAR, I'd test it at half depth to get an average for the length of the blades. My Manatee Grass has grown quite tall to reach brighter light. I'd agree a six inch substrate is good. You might be able to go a touch shallower if needed. I'm anti-hermit but that's just me. The emerald crab I tried ignored bubble algae, ate desirable macro algae and ignored the grasses.

I found fresh water planting methods worked great with grasses and macros. My concern would be combining fertilizers with seahorses. Dechlorinated tap water may be fine for the plants but probably not with seahorses. If you are serious about salt water, get a good RO/DI setup.

Good luck with your project!