First thing to note: These are not good to have in your tank, but they are not horrendous either!
If you are seeing tiny star-like organisms on your glass, they are likely to be Hydroid Jellyfish. They sort of look like very small Aptasia, or a snowflake or even baby star fish, they are not really much bigger than the head of a pin (1-2mm max).
Hydroid Jellyfish tend to be found in newer, less established tanks. You will often see a bloom of them, then they will just slowly vanish over time as your tank ages.
These little Jellyfish are also known as Cladonema, Hydromedusae & fixed Jellyfish.
For the most part, these little Jellies are harmless, but they are Jellies, and as such can sting. Though you won’t feel their stings, your fish will. The stings will not hurt your fish, but it is probably uncomfortable for them.
How do you control the numbers?
Some reefers will claim that the certain species of Wrasse will eat Hydroids, but there is no real evidence of this.
Population numbers tend to dwindle naturally once your tank is more established.
Do they have any predators?
There is no reliably known predator for Hydroid Jellyfish, but some people have seen some success with the following:
- Six Line Wrasse – Be careful if you choose to go down this route, Six Line Wrasses don’t have the best reputation for playing well with their tank mates & there is no guarantee they will eat the Hydroid Jellyfish.