I have been reading your website's FAQ's and have found them helpful. I have just recently purchased a mature saltwater tank. It is a complete glass tank with overhead arcadia lights. It holds 110L of water and has at present about 10-15kg of live rock, which has green fronds, which I am assuming is algae of some kind!
Also inhabiting the tank is a black tailed humbug, a midnight angelfish, and also a teddy bear Crab. Which leads to my first question, - should I remove the crab from the tank? I have seen it a few times at night and it does not seem to bother with the fish in the tank nor is it bothered by the fishes.
I tested the water in the tank 2 days ago and most of the readings are OK - SG 1.022/Salinity 30, pH 8.4, Nitrite 0, Ammonia 0, but the Nitrate is reading 160 ppm, and would like to know if there is an effective way to bring nitrate down? Or would the tank benefit from having a cleaner fish added? When I purchased the tank it had only 50% water in it and I topped the tank up with salt water purchased from my local aquatic dealer.
My other question would be what would be the best fish to add as in the future I would like to turn it into a reef tank? I was thinking about adding a Yellow Tang and a Clownfish, but not sure if these would go with what is in there now.
Thanks for the email, and from what I gather you're quite new to the marine hobby, and before mistakes are made, we need to clear up several aspects.
You say Specific Gravity (SG) is 1.022/Salinity is 30. That's fine for a fish only aquarium, but not one containing invertebrates (the crab is an invertebrate). Suggest its SG be upped to about 1.025 if you want to kept the crab or add other inverts.
As for a black tailed Humbug, that is a very aggressive damselfish, and I doubt any other fish could be added without it declaring war on the newcomer. It would wipeout a clownfish! And since 110 liters is only about 30 gallons, a Yellow Tang would be a poor choice, as it requires at least a 75 gallon or larger aquarium.
The Centropyge nox, which is called the 'Midnight Angelfish, is fine for this size system. Yet needs to be fed correctly, as it's a browser and in a small tank will not find enough of a natural supply of food.
The Teddy Bear Crab is an interesting addition, but anything with a fairly large claw cannot be trusted, especially if it becomes very hungry. And when fish sleep at night, they are easy targets for predators. Therefore, either keep the crab well fed, or remove it.
I'm fairly sure you are reading the nitrate result incorrectly. There are two types of test kits on the market for this compound, and suggest you read its instructions and recalculate the results as needed. Then get back to me and we can go from there.
As for adding corals or more fish, suggest mulling over the above comments as to what you now have, then also acquiring an understanding of proper calcium, magnesium, and alkalinity levels and how to maintain them before moving onto adding some corals.
Hope this helps and keep me posted.