Ive had it ruining for a while, almost a year but i just got these clownfish recently. Ive tried directly the red slime algae 3 times in the past and i removed it again today but it always seem to come back. My water parameters are all good but i dont have a reef test kit ,i only have the saltwater test kit. I dont plan on adding corals until i get that test kit and some more supplements. Although my local fish store states i cant add some coral when i add the supplements i already own i want to be on the safe side. I currently using some pretty simple stuff as my filtration like filter sponges, ammonia remover, carbon, and some biomax.
Once you get that algae to go away then you can start looking for some corals. Keep in mind a nano tank gets difficult at times because of how quickly things can change in a small volume of water. But the more you learn the easier it will become. You are going to want to get both kits for testing.
If you can afford it I would recommend getting the nitrate pro kit from Red Sea it’s easier to get a more accurate reading . I too started off with api kits. But now I use Red Seas pro nitrate, magnesium, calcium tests
And for phosphate and alkalinity I use the digital Hannah checkers. However if I do test for ammonia and nitrites I do use api.
Like I said try to manually remove as much as you can of that algae and change out your filtration media then do a couple days without lights on. If not you might have to get a chemical product that kills of cyano.
As for corals you could start of with
Some soft corals like Xenia they are very hardy but keep in mind they eventually grow like wild fire
You could get some lps corals like favia or chalices they do grow slow.
Some other good soft corals to start of with are your main stream zoas like Rasta and sunny d’s
Do not get a starfish lol
Sand-sifting starfish are great at first but they tend to starve because of how quickly it eats its food resource in the sand.
As for urchins your tank is not big enough.
You will want to get a couple of hermit crabs and snails (cuc)
And yellow clown goby would be fine in your tank keep in mind they perch on corals and some corals don’t like it lol most of the time you will be fine with a clown goby and only get one