Welcome back! I too am just jumping back in after a nearly 17-year hiatus, there's a LOT that has changed since we were in.
LED's back then were a nice idea, but barely there in the spectrum, very little extra functionality, and
spendy. Nowadays, T5's and MH's are effectively
passe because LED's excel against them in nearly every way that matters, tho some people do definitely run hybrid combos. Wattage requirements are a LOT lower compared to the old formula we used to use, but for premier-brand lights (Like Kessil or Tunze, AI, etc.) with a lot of functionality, you can expect to spend $5 - $7 per watt.
I have also been advised not to buy used LED's more than 2 years old - their spectrum DOES fade over time, with use, and eventually requires replacement of either the entire fixture, or the diodes (not all can be replaced, and you'll need to be handy with a soldering iron even if they can be: it's not "plug and play"). People tend to run them at max power which contributes heavily to that shortened life-span. Certain brands (e.g. Aqua Illumination, aka "AI") also have a reputation for burning out their diodes (particularly red ones) as well as melting their plastic lenses.
ULNS (Ultra Low Nutrient Systems) are also a thing of the past... the community has since realized that trying to keep systems like that was what was causing a significant portion (if not the outright majority) of dino, algae and bacterial blooms, especially in mixed-reef or non-FOWLR systems. Nowadays, the guidance is that nitrate should be between 2 - 10ppm (depending on your system), and phosphate should be roughly 1/10th of nitrate - neither should ever be 0.
People are actually ACTIVELY adding small amounts of pure ammonia to their tank
on purpose, because the corals actually need some. Also, vodka or sugar (to dose carbon).
Dark and fish-less starts... aren't a thing of the past, per se, but there are other ways now. I'm planning a live start for the 50g AIO I'm working on standing up now: water and FritzZyme (bacterial booster) along with a coralline algae booster on Day 01, three to four hardy starter corals on Day 02, two to three hardy starter fish on Day 03.
Torch corals have gotten ridiculous. They used to be one of the cheapest corals in the hobby (next to mushrooms and GSP), now they're one of the more expensive "common" corals. Of course there are many even pricier (notably bouncing mushrooms) but... you might want to wear a grounding strap before looking at prices, they might shock you.
Copepods have been found to be critical to a healthy system start, and that having a strong and thriving pod population (with good species variety, not just one kind) from the start should dramatically shorten all the traditional "ugly phases" of a new tank. The phases will still happen, but if you've got a healthy balance between a couple of fish, a few corals, and lots and lots of pods, those phases will be shortened to about a week or so each and won't require heroics or chemical interventions in most cases.
There's a lot more, but that's what occurs to me off-hand.
You may want to look at the
Manta Systems website, run by our own
@Tamara Marshall who regularly contributes very informative articles on a variety of reefing topics.
