SW systems are a completely different type of system, compared to FW.
Now this post is going to sound a bit like I'm tearing apart someone because they are new to SW systems. This is not the case. I want you to have a great tank, but your doing a lot of things wrong.
The Rena XP2 is an ok filter system, but it is a canister filter. Canister filters in general are less than ideal for SW systems. In addition, I would consider an XP2 to be a bit undersized. Manufacturers tend to be very "optimistic" about how large a tank their equipment can handle.
Canister filters in SW tend to become dirt traps. This is because they usually don't get cleaned often enough. At that point they tend to create a lot of nitrates, which is not wanted in a SW system, and can often lead to massive algae outbreaks. Generally a canister filter in SW should be cleaned weekly.
This doesn't mean you can't use the canister filter, but there are better ways.
Biological filtration in a SW system is best done by using live rock in the tank. The amount of biological activity that can be maintained in a canister filter is limited. You do not mention if your using any live rock or not. Typical amounts for your tank would be about 23 kg to about 34 kg. Yea, this stuff can get expensive. There are alternatives, but that's a whole other subject.
Your tank has been set up only for 8 days. It's far too new to be adding fish or other livestock. While all aquarium systems go through an initial cycle, in SW it's critical. This is most likely why you have seen fish losses. You need to have the tank set up, usually with live rock, introduce some sort of organic matter to decay (most use a single uncooked shrimp purchased from the sea food store), and wait. You'll see ammonia go up, peak and then fall, followed closely by nitrite. When both get back down to 0 the tank is initially cycled. This usually takes 3 to 4 weeks. There is no short cut. Yes, you will need your own test kits.
The brown algae you are seeing is most likely a diatom bloom, and usually will pass quickly. Keep an eye on the tank, because you don't want outbreaks of other unwanted algaes.
Addition things I recommend -
If you don't already have one, get a good quality skimmer. I would recommend something like an AquaC Remora Pro, or similar skimmer. Yea, I know, yet another item to buy. You need not get that skimmer, but use it's price as a guideline as far as how much you'll need to spend. There are a lot of junk skimmers out there, especially on the low end. If your not sure, ask about your intended purchase first.
Get yourself a few good books on state of the art SW reef systems, even if you don't plan a reef system as such. Then read and study them, so you have some idea about what your are doing.
Here are two to start with -
The Conscientious Marine Aquarist
by Robert Fenner
The New Marine Aquarium
by Michael Paleta <---This book has an especially good section on fish suited to someone starting off in the hobby.
This is information that you can not easily obtain from the net. While it's out there, it's all over the place, and there is a massive volume of information. You need something to get you started.
Join one of the forums devoted to SW systems. I don't usually recommend one forum on another one, but since this is a planted tank forum, and not really involved in SW systems I don't feel bad about doing this. For someone starting out, you might try Reef Sanctuary (offsite link) - http://www.reefsanctuary.com/
They are usually helpful to beginners. Plus you'll get to read all my "erudite posts"
Reef Central is the largest forum, and there is a lot of high end information, but it's sometimes noted for being harsh on beginners (offsite link) - http://www.reefcentral.com/
Hopefully, this information will get you started.