How to stock my 20 gallon?

So I set up a 20 gallon nano reef tank a few weeks back. I got the whole setup already established. I've monitored it for a few weeks it looks to be fully cycled and the water parameters are all good. The thing came with a maroon clownfish and that guy seems to be doing well. I've read maroons clowns need larger tanks but there's nothing I could really do about that now :/
The tank has an emerald crab, a pincushion urchin, brittle starfish, hermit crabs, pistol shrimp, and lots of snails for a cleanup crew. I'm kind of worried about the urchin and crab but so far they seem to e minding their own business. I also have some small corals and polyps. Cute little things. Lots of live rock. About 35 lbs. Then there's three powerheads and a filter. (and of course all the other things like thermometer, hydrometer, heater, etc)
ammonia 0
nitrite 0
ph 8.2
nitrate 10
temp 78.5
I do have a calcium supplement but I don't wanna use that because I don't have a calcium test kit.
does instant ocean sea salt provide the nessecary elements corals need like calcium, magnesium, strontium & iodine?
right, now on to my actual question...
What do I stock it with?
I want at least one more fish to put in there. If I could fit 2 more that would be awesome. I was thinking watchman goby, but I don't have a lid.
which brings me to another question
because I don't have a lid, water evaporates quickly, so do I replace it with freshwater or saltwater?
I also wanted to add some peppermint shrimp. can I do that?
I think I might have aiptasia and I heard they ate that
can cleaner shrimp go with pistol shrimp
back to the aiptasia, it came with the live rock, but when I first say it I thought it was another coral, because I didnt know what aiptasia looked like. then it started popping up and I say little tiny baby aiptasias popping up everywhere. they don't seem to hurt my corals. in fact, one of my corals (its like a maroon colour with little green bubble things) is squished up right in between two of them and looks pretty good. Are aiptasia really that bad? are there any corals that look like aiptasia?
thank...lots of info here.

 

Aiptasia can sting and kill your coral. You probably could get away with getting a fish that stays slow I can't remember if the watchman goby stays small or not off the top of my head. One I kept in my biocube was a bicolor blenny which I really liked he helped take care of the algae problem.

 

Aiptasia is very bad for your tank it will sting corals and fish and it reproduces very quickly. Peppermint shrimp may eat it. First off if you are going to run a saltwater tank it is better to use real salt instead of the synthetic salt I think you are usinjg. Using the real salt will greatly benefit the corals. The maroon clownfish is in a tank that is to small for him. Maroons need at least 30 gallons to live in. He will only grow to a certain size in the tank, but as a result his life span will be greatly decreased. I would recommend taking him to the LFS and selling him and buying a smaller clown fish. Right now everything seems good and the cycle seems to be done, but you most likely will hit a major algae problem and this can be bad for the fish depending on the algae you get, this is normal for new saltwater tanks. Don't try adding anymore corals for now until you have had the tank for at least 6 months even then don't start adding corals that are tough to keep. Start with very easy corals and begin making your way up to different corals when you feel you are ready. I will be posting again later.

 

When replacing water that has evaporated from the tank, always use freswater for the top ups. Beacause salt does not evaporate, you will just be raising you salinity. Peppermint shrimp can go in your tank but they might pick at your corals and steal food from them. If they are a problem you can always remove them. The peppermint might not get along with your pistol shrimp. I read up on the Aiptasia for ya. First off, don't try to cut them off rock or rip them out, this will create multiples. The pieces will land and form a new Aiptasia. If they are on the gravel or sand, syphon them out and throw them out. If they are on a rock try rinsing the rock in freshwater then remove the Aiptasia. Do not try to remove them with your bare hands. I heard they sting. Peppermint shrimp will eat the Aiptasia so you could try that. You don't need cleaner shrimp for eating the Aiptasia when the cheaper shrimp will eat it. If you want more info on Aiptasia, try this link:
As for the Watchman Goby, he should be fine in the 20 gallon. The info page on this site says 30 gallons but the goby only gets to be 3 inches in size. Also the goby may bond with a Pistol Shrimp. They will help eachother out and live in a cave together. I think that's kinda cool! I hope I helped and good luck with your tank. Takes things slow and research and everythings will turn out great.

 

where can I get real salt?

 

I would get a sea salt mixture like oceanic sea salt, it does have trace elements that are needed by your aquarium, don't use "real"" table salt, get a good brand of sea salt from the local fish store, also what is the light setup of your tank ( will be important to know for future coral selection) also I know that maroon clowns are beautiful, but they do get too big for your tank ( I inherited one with a 20g, the most beautiful fish I have ever seen, and so wanted to keep it, but it was really clear it was not happy in such a small tank so I sold him back to local fish store so he could have a happy home) ocellaris clowns would go in a 20 gallon and you could get a pair, I would consider getting a lid, it will cut down on evaporation and help keep your salinity more stable as saltwater fish and especially coral are very sensitive to water paremeter changes . a cardinal fish, or a blenny or goby or various damsel fish are some choices you could have for your tank
best of luck

 

Any of your LFS will sell real sea salt. Just ask them for non synthetic stuff. It costs more but you need it for reef tanks.

 

You can use a product called AptasiaX to get rid of the aiptasia rather than confuse yourself with different types of shrimps - However the Peppermint is a good all round cleaner.

You will need RO (Reverse Osmosis) or DI (De Ionised) water for top ups and A good marine water (RO or DI with salt) for water changes.

Most local fish stores (LFS) can provide these or alternatively you can buy your own RO unit and a good grade reef salt to mix your own.

The Maroon clown will certainly benefit from a larger tank, so finding him/her a new home is not a bad idea.

A pair of common or Black clowns would be a nice addition to a tank this size.
A yellow watchman goby is popular choice. (sand dweller)
A scooter blenny or Mandarin. (rock dweller)
A flame Angel or Coral beauty. (midwater)
Having a selection of different tank level fish will give you a bit more space to play with.

There are some smaller goby species that you could look at such as the neon or dracula.
Damsels are OK provided you get a docile species, but personally I stay away from these nasty fish.