Ello!
Just thought I'd add some stuff, not sure if it'll help or not xD
So you want to set up a saltwater tank? noice! If I was you I'd start with a FOWLR system, then after it gets established you can try some corals. I am more or less new at SW fish keeping, had plenty of failures so I'd hope to help you not make the same ones xD. It CAN be an expensive hobby, I have basically a 0 dollar budget most of the time. Making alot of things yourself can really save money, but at the same time it's good to not skimp on things that matter. Water pumps, heaters, lights, and skimmers are all things that should be decent at the start, remember things can be upgraded later
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I skimmed over other people's posts and didn't see a ton about filters (Could have just been too lazy to read deeper...) Anyway, I'd recommend you think about making your own sump type of filter. Research them on Google, they are really pretty neat. Not a necessity at the beginning but it's definitely good to think about. (for a 20g tank you could probably easily do a 10g sump system)
One thing that saved me is the fact my LFS has a "used" equipment section, I bought everything but the heater and main water pump from there. Lights, skimmer, recirc. pump, and even an auto top off system! All at a fraction of the retail price.
Check it out if they have one!
BIGGEST MISTAKES I MADE:
1) not using pure water. I'm paying for it now with lots of algae and nutrient problems.
2) rushing and buying a lot of stuff fast. the LFS by me had a sale and I spent 200 bucks on fish (thought it was a good deal) Anyway, a massive ick outbreak killed them all shortly after. Really really depressing learning experience. Which brings me to #3...
3) not quarantining new fish. As stated above, lost a massive amount of money not taking it slow and thinking smart. Even if the fish looks fine, it may be carrying a parasite or something.
4) starting a SW hobby while living at home. I'm only 17, but I've loved fish since I was real little. Started with a betta, graduated to a 75G (soon to be) reef. My parents yell at me ALL the TIME for normal things such as mixing new saltwater for a water change or messing around with plumbing and the equipment, and especially spending money on the fish (even more so after the ick incident)
This fish tank is more than just a box of water and fish to me, it's a responsibility to keep the fish happy and healthy. I learn a lot just sitting there watching the fish and inverts. I'm definitely no expert, but I think I'm doing pretty well now.
So, I hope that helps somehow.
(probably just bored you death... :zzz: )
Edit: Also, no matter how much you research, you still need to know more. Learned that personally hundreds of times.
Always try to learn something new! Knowledge in this hobby is the biggest investment.