You will have to read on the internet bro... This is heavier cost than FW
, depends on what you want.
For corals keeping you need high lighting including actinic and white colors bulbs/MH
For fish only you can go with what light you want
Biological filtration is most of the time done by the Live Rocks, some hightech setup include bios medias into a sump.
Do you want a sump ?
Mechanical filtration is usually done by protein skimming, with protein skimmers. Regular filters pads need to be cleaned each 1-3 days, depends.... Maybe more...
Temperature must be 24-27C°, PH 8.0,8.4. Saltwater must be prepared before WCs, must buy an hygrometer to make sure you have good salinity...
for a 20g, two clownfish and some easy corals like frogspawn, hammers, torchs are good for beginner...
You need a cleaning crew (optionnal but very very very helpfull) like snails, shrimps or stars like brittle stars.
QT tank is optionnal but very usefull... You can start by trying a 20g setup, then upgrade to bigger setup if you like it and keep the 20g for QT....
How much gallon do you want ? 20g cost 300-1000$, 90g cost around 2000-15000$
Price/gallon is exponential.
Personally, I started a new 20g, with all new equipement, hightech tank without sump.
20g tank + R420R 60w led
light (300$) + 50g HOB
skimmer + LR
+ powerhead(s) (I suggest two small like 240GPH) cost me around 1000$ only for equipement.
Add 50$ for two clownfish, 15$ a tiny frogspawn, 20$ zoa frag, 25$ birdsnest that's now dying because I didn't know for light acclimatation...
That look like this:
It's normal you don't see corals on the first pic, I didn't bought them when I took picture. That's my 20g SW
and 29g FW