Tiny Sand Fleas in Marine Tank

Question
QUESTION: I have a large amount of very small brine shrimp looking sand fleas/fruit fly
looking oranisms in my salt water marine tank.  It looks like they have built a
cocoon on the back wall of the tank (two small white sacks). I believe they are
eating the algae on the back wall and may be attacking my zennia. I have had
the tank approx 5 months. Water is good.  Some say this is natural and
represents healthy live rocks sprouting life.  I fear it is more sinister.  Do I
need a shrimp to eat such things?  Any idea what this may be?

ANSWER: HI Jeff. What you have is an exploding arthropod/copepod population. This is good for your tank and will not harm your xenia. These organisms are part of what makes up the plankton in the ocean and is crucial for all filter feeding corals and other invertebrates that you may have. As well as providing food for many of the creatures in your tank they also eat some algae and detritus that can build up in your tank. They also will bury into the gravel where they help to keep small spaces in between the particles where some oxygen can flow and help the gravel from getting stagnant areas. As for the white sacks I am curious if you have any snails in your tank? Snails will often lay eggs in an aquarium and they can look like small plastic-looking pouches that will stick out from your tank wall or they can be gelatinous with white dots stuck in the jelly like mass. You won't see the eggs laid by the copepods or arthropods as they will be even smaller then the creatures are themselves. Some people go as far as to culture these little creatures to keep a supply of them in their reef tanks(me being one of those people). The problems you may be having with your xenia are most likely due to the fact that your tank may be too clean to house xenia. This coral prefers water that has a slightly higher amount or organic waste in it then most corals like. If you need more info on xenia let me know. I have had xenia for many years and am very experienced on its care. It is one of my favorite corals and I also propagate it for resale.

By the way you may want to consider getting a mandarin blenny. These little fish will thrive in a tank with a heavy copepod population. They will eat so many of them that they may not be able to reproduce fast enough to keep up with the fishes' feeding needs!

---------- FOLLOW-UP ----------

QUESTION: Great info, thanks.  You may be right about the snails. I have two small snails
but see a LOT of white dots on the back of the tank wall.  Should I expect an
outburst of snails?  Regarding the Zenia. I only have a blue damsel and a few
crab/snails at this time.  We may be to clean for the zenia.  What can I do?

Answer
Hello again. If your snails are laying eggs in your tank they will probably do it often. Look really closely at those dots. Snail eggs are either large and protrude from the tank wall or are encased in jelly with a bunch or whit dots concentrated in one spot. If you are seeing lots or random dots on the tank wall those may simply be very small feather dusters. If you look really close you may be able to see a spiral shape to them and if you look even closer you may see a red feathery looking protrusion. Very hard to see unless you look really closely. There are a few other things that they could be and all of them are completely normal in a saltwater tank. If they are snail eggs you will not see an outburst of tiny snails in your tank. They hatch and become a part of the plankton and either get eaten by any filter feeding inverts. or sucked up by your filter. One or two may survive but even that is rare.
That's the funny thing about xenia. It lives in some peoples tank and not in others. Your tank may just also be too new as well as too clean. Xenia has a very low light requirement and actually uses nutrients from the water almost like a plant would. It does not do good if you are using any iodine additives and it will not feed off the plankton live or liquid additives. Xenia loves a strong water flow, pH on the higher end of acceptable and salinity of about 32-34ppt. Other then that the tank conditions may just not be right for this coral due to the newness of your tank and the lack of dissolved nutrients in the water. There is no additive to use to help with this. There is still allot of unknowns when dealing with xenia.