o. mercatoris


Aug 24, 2009
i plan to get a o. mercatoris. i'm new to tonmo but i'm sure i've already bored you all with at least one of my threads. this will hopefully be a little better. i'm just going to say my basic plan what i need and how long i think it will take me. and please give me any suggestions, comments or problems you have with it.

right now i have a 25g tank i thing if i provide a couple of hiding spots i can hold 3 o. mercatoris(es) sugsesfully. On the tank i will have a protein skimmer, a carbon filter, an air stone, and a UV filter (all behind a transparent divider so the octopuses can't dismantle them). I will feed them 1 live fiddler crab every day and set the temp on my heater to 75F. Every 2 weeks I’ll change 1/3 of the water (using instant ocean as salt) and test the pH and NH3. I’ll add water stabilization solution and buffer as needed and that’s about it.

Please respond to me. i need all the help i can get


Certified Ceph Head For Life
Staff member
Sep 4, 2006
Gainesville, GA
You don't need (and it is potentially undesireable on a ceph tank) the UV filter.

Unless your room temperatures go below 70 degrees, you will not need or want a heater.

I am not sure what you are referring to when you say "stabalization" solution. What is it you expect to stabalize?

You do not mention live rock. I feel LR is absolutely necessary for filtering any marine tank.

You will want to check your salinitiy as well as test for (as Dustinh lists) nitrite and nitrate in addition to ammonia. After you cycle the tank for three months, if you detect nitrites, something is wrong and you will need to do very heavy water changes until the situation is corrected (usually occurs when a new tank is not fully cycled but decaying food hidden in the back of the tank or an undiscovered dead animal may also overwhelm the biofiltration). If your nitrates begin to rise, you need to increase your water changes and be sure you are actively cleaning the bottom substrate and rock work.

You will need to top off your tank with fresh water daily. The freshwater should be the same PH and temp as the tank water.

In a small tank with high bio-load, a once a week water change is better and I change a full 5 gallons weekly on our 25 gallon nano (mantis shrimp tank). Water changes are more than just swapping saltwater for saltwater. Before/as you remove the water, you need to stir up the bottom substrate and vacuum the dirt and trapped particles (I use a bamboo skewer and my siphon).

It would be best to use distilled or RO/DI water for both your water changes and mixing your salt. Your salt should be mixed in at least 24 hours in advance.

Mercs are not overly prone to escape but some kind of top edge covering (at least an inch) over the top will be needed to keep them in the tank. Keeping an air space of at least an inch (two is preferable) is also recommended.

It would be helpful if you would keep your posts and questions in one thread and not start a new one each time you need help. Having all your questions together will help you and others see the responses for a new start-up system.

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