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Purchase this book....for one. Marine test kit. And Hydrometer to measure salinity levels.
Now remember salt doesn't evaporate with the water...so top off with clean water that is nitrate,phosphate, and metal free....most recommend RO water since most municipal water supply is horrible. Most not all....
The first thing here is you describe hair algae or cyanobacteria ....without pics we can't tell. Turn on the lights....is it dark blood red with bubbles and spaghetti strands...or like a bad green afro?
First things first. Major clean up.
You need to prepare 30 gallons of salt water or go to your local marine store and see if they sell it.
IF this is that deep marroon slimy spaghetti like algae with micro bubbles under the sheets...it is very likely cyanobacteria...do not brush it or it will spread ....siphon it all out......
IF its the green hair algae...hand pull all you can out..scrub rocks out in buckets of water you siphon out ..not tap water....use the water you pulled out to clean rocks...scrub all you can off.
Be very sure you have the temperature of the new 30g as same as the tank also before you begin.
You need a huge water change first off....
To maintain this tank from here on out. They don't have to purchase HOB skimmer at this time.. And I never recommend canister filters ever to anyone in this hobby. They trap detritus material...which in turn does not get pulled out of the water by skimmer or water changes.. It acts as a factory for nutrient import into your tank. That is not something I ever would recommend to marine. We have to keep our nitrate levels much lower vs FW......we also have to keep phosphates at 0 if we can. Mostly reefers do this..>FOWLR is more forgiving....I see why people may say get a canister. I Say Do NOT.
Your best filter at this time is good maintenance. Weekly water changes...Since you don't skimm ...Your best bet is to do 10g-20g water changes weekly. Watch those test results before and after changes. This will give you an idea if your changes impacted them enough.
40g isnt huge tank...and most nano tanks do well with simple big water changes.
IF you decide canister filter...at the least pull out all media...and replace with crushed live rock. About the size of plumbs and apricots. At the very least it will have both bacteria needed in the breakdown process. And weekly clean that out by dumping the water inside into buckets and releasing any trapped detritus. (i still don't recommend this item)
Good luck....do use a cell phone and take pics....and try to get the up here for us......we can further advise with better information.
PS: Something I didn't know vs FW Fish. Marine fish drink their water vs FW....FW fish pass it through....I learned this from that book. So having a more pristine environment for Marine fish is best since you dont want Mr.Fish drinking his own poo... =p