Strange guests in my tank

[BrucePerens]

Hi,

My 10-gallon tank has Opae, pipipi, dwarf hermit crabs, and trumpet snails. Today I am seeing some sort of guest. They are too small to be opae fry. I only see light-colored dots. They are phototropic, and arrange themselves in a tight column about 1/2 inch around and running from the surface of the tank under the center of the light to about 3 inches deep. Within this column they are moving up and down. There might be a few hundred. I saw another column like this a few weeks ago.

The 10-gallon tank is heated to 76 degrees and lighted with an LED panel on a timer and filled with water a bit less than half ocean salinity. On the bottom is crushed coral and a lot of shells of various sizes for the hermits. There is a pile of volcanic rock taking up one side of the tank. The only aeration is a bubbler buried under the crushed coral which is under the volcanic rock pile, which gives a bubble every 30 seconds or so. There is no filter. There might be over 100 opae (they mostly hide), 10 pipipi, 10 hermits.

What could this guest be?

[Varanus]

Other users have mentioned things like these in the past and the general consensus is that they are young copepods (a terms that includes a diverse array of tiny crustaceans) and that they will typically die off due to the low nutrient level in a supershrimp tank.

[BrucePerens]

the general consensus is that they are young copepods.

OK. About the only thing that's gone into this tank in 6 months is Fluval Veggie pellets, Instant Ocean salt, and RO filtered water. Are the pod eggs riding in on one of those?

I can't imagine there's enough free floating algae for them to eat, so yes they would probably be gone in a few days.

[Varanus]

the general consensus is that they are young copepods.

OK. About the only thing that's gone into this tank in 6 months is Fluval Veggie pellets, Instant Ocean salt, and RO filtered water. Are the pod eggs riding in on one of those?

I can't imagine there's enough free floating algae for them to eat, so yes they would probably be gone in a few days.

I don't know if its true of salt water species, but some freshwater ones are known for having eggs that can stay dormant for extended periods, so its possible the copepod eggs have been in the tank a long time.

[Corucia]

I had the exact same situation in my 7 gallon opae tank back in ~April. It appeared to be a "bloom" in the population of small copepods that clustered to one or two dense columns and exhibited the same up / down almost cyclical motion. It disappeared in about five days and the tank is back to its seemingly normal mix of copepods in much lesser numbers now. At the time, I think the population bloom coincided with the increasing temperature in the tank as the winter months transitioned into spring. I also had somewhat of an algae bloom shortly thereafter (clouded the water pretty meaningfully) which lasted for about a month and then disappeared as well. Curious if you might be seeing a similar algae / clouding of water effect.