If you want to keep corals, that light (IMO) is definitely underpowered. Since you mention you're on a budget, I'll let
@Reefing102 make a lighting suggestion since I'm not really up to speed on LED lighting (I'm still using 1990s/2000s Metal Halide lighting). With metal halide, we looked at watts per gallon to determine which lighting to use. With LED it's more about PAR values and (since I'm not using LED), I've really not researched that. That said, most of the LEDs can be dialed into a certain intensity. In other words, you can dim them.
The heater and filter should be fine since you are only looking to keep fish and easier to keep corals. You'll want to find out which filter it is because you want about 10 times turnover of water movement in your tank. So, with a 21 gallon, you want around 200 gallons per hour of water movement. Wavemakers are really nice to have as they not only move water but create varying currents to help keep detritus stirred up and allow food/nutrients to flow to the corals. For budget wavemakers, look at Jebao and Hygger. Both have good reputations and are relatively inexpensive.
Also, I'm not sure what ammonia, nitrite and nitrate removers your brother had but, I'll hazard a guess they are designed for freshwater only. For example,
zeolite is used in freshwater to remove ammonia and does not work in saltwater.
The freshwater test kits and pH adjusters will not work for saltwater. So, you'll want to get new test kits. I recommend Salifert for ease of use and being relatively inexpensive. Another good brand of test kit is Red Sea. To start, you'll want to test ammonia, nitrite and nitrate to monitor your cycle. Once the tank has cycled you can then pick up other test kits. Since you only want easy to keep corals, you should be okay with just testing pH and nitrate going forward. You'll also want either a
hydrometer or
refractometer to measure the salinity of the tank and water change water.
Source water (tap) is another consideration. Most people who keep reef tanks use a reverse osmosis deionization ("RODI") system to make water for their tanks and topoff. RODI systems basically remove 99% of impurities in the water. Many people do have successful tanks using just tap water with a
chlorine remover but, more prefer to be safe with RODI. An alternative to getting and RODI system is, some aquarium stores sell RODI water at least here in the US. Some grocery stores here also sell RO (without the DI) for drinking water purposes. Just don't use distilled water as some distilled water in made using copper piping and copper is toxic to invertebrates and corals.
I think I've answered all your questions except for, "Lastly what should I have before I put water in the tank?"
1. Tank - You already have
2. Heater - You already have
3. Filter - You already have
4. Thermometer - to keep and eye on your water temperature (if you don't already have one)
5. Test kits - At a minimum to start, you'll want to get a hydrometer/refractometer, and ammonia, nitrite and nitrate test kits
6. Salt - Many good brands out there. I currently use Instant Ocean as it's readily available and fairly inexpensive (I buy the 200 gallon mix boxes when they're on sale.
7. Rock - As mentioned earlier, you can use
dry rock, live rock or a combination of the two. Since you're in Oz, your options for live rock are likely limited to Australian live rock. I don't know the price of this in Australia but, in the US Aussie live rock is pretty expensive ($25/lb compared to US aquacultured rock at $7-$10/lb). I personally prefer live rock because it adds a lot of life to the tank like macroalgae, small inverts, coralline algae, and many times has hitchhikers like shrimp, crabs, snails, starfish, etc.
8. Sand - You'll want a good aragonite based
sand. How much is up to you.
9. Protein skimmer - Optional buy since you're not going to use a sump, you'd need a
hang on back skimmer
10. Wavemaker - Optional but, again, depends on the gallons per hour of your filter
Hope that help and keep asking questions, we're here to help.
