My brackish tank has been running for about 8 months. Just today I noticed on the tip of my thermometer a colony of polyps growing on there. The area it covers is roughly 1.5cm square area. The size of each polyp is under half a millimetre. When it has it's tentacles out, total length is about 2mm. They are white in colour. They retract every time I touch them with my clips, and pop back out a few seconds later. In the past I thought it was just a patch of algae, so I ignored it.

Can anyone confirm what this might be? If it's a polyp does that mean I have small jellyfish in my tank? I don't mind them growing in there, but I want to ensure that they will survive in there. I would like them to be growing on a rock rather than the thermometer... Is there anyway to make them grow on a different surface?

I can't take a photo of the colony because my camera is broken at the moment. Plus they're probably too small to capture.

These are likely hydrozoans of some sort; the genus Hydra is the best known, but it is exclusively freshwater as far as I know. Other genera, such as Obelia, do occur in brackish water.

Cheers, Neale

Thanks for the info Neale. It eats live organisms, which is great because I have plenty of small flatworms, brine shrimps and many other things which are difficult to see.
But if I left it to grow, will it spread easily throughout my tank and become a pest? It kinda looks difficult to remove. Although I would really like to see free-floating medusa in my tank.

I just discovered that I have a large colony of hydrozoans growing on my water heater. But so far none are growing on the glass of my tank.

No chance at all of them becoming a pest. The free-living medusae will get sucked into the filter, so any growth will be vegetative as the colony spreads across flat surfaces.

The idea of a freshwater reef tank is intriguing though. Certainly such communities of fresh and brackish water invertebrates exist, but they aren't traded, so what you find in tanks is very much down to luck.

Cheers, Neale