• Looking to buy a cephalopod? Check out Tomh's Cephs Forum, and this post in particular shares important info about our policies as it relates to responsible ceph-keeping.

Horseshoe crab?

Oceanic Art

Jan 13, 2017
DartmouthNova Scotia
i have a horseshoe crab in my octopus tank, I'm not really dead set on keeping him in there if it's dangerous to the octopus, however the octo doesn't seem interested, at least not yet. I know they have blue blood because they use copper in their blood instead of whatever it is we use that makes our blood red I can't remember. Anyway, would this be an issue should the octopus decide to taste it? I've read they don't really have the means to harm an octo with its tiny claws so I'm more curious the outcome if the octo decides to eat the crab. Any info would be great
Octopuses also have blue, copper based blood :biggrin2:.

I am not sure if an octopus will attempt to eat a horseshoe crab or not. I suspect a vulgaris would try but not at all sure about a dwarf. They are great sand sifters but don't do all that well in a tank for long. I've kept one in a tank for over a year with successful sheds but their natural lifespan is around 10 years with a size that is not suitable for most tanks. The biggest worry is that it would die and not be discovered under that sand, creating an ammonia spike.
Octopuses also have blue, copper based blood :biggrin2:.

I am not sure if an octopus will attempt to eat a horseshoe crab or not. I suspect a vulgaris would try but not at all sure about a dwarf. They are great sand sifters but don't do all that well in a tank for long. I've kept one in a tank for over a year with successful sheds but their natural lifespan is around 10 years with a size that is not suitable for most tanks. The biggest worry is that it would die and not be discovered under that sand, creating an ammonia spike.


  • IMG_6980.JPG
    1.1 MB · Views: 124
  • IMG_6989.JPG
    1.5 MB · Views: 108
  • IMG_6990.JPG
    1.8 MB · Views: 103
Here is some pictures. I've reomoved all fish from the tank and brought them home. The pictures should show rocks that have been sold to me as "live rock". I'm still new so I'm not sure about all these things. I have a thing called "amo-lock" for ammonia and "bacto reef balls". Should I add chemicals to the tank to alleviate the stress on the octo or simply hold tight and do regular tests? If the tests show high what is my next step? Sorry to ask such beginner questions, my goal is to learn and become more kowledgeable and keep a healthy octo
LOL, the purpose for our keepers' journals is to ask, answer and share so no questions need apologies.

Live rock and cycling saltwater tanks is not a defined science but often a pet store has no real clue about the condition of the LR or what advice to give on cycling (the exceptions are usually owners and employees who keep saltwater at home. Most live rock I have seen in pet stores is kept in a tank without animals. Detritus from the animals creates the ammonia that feeds the bacteria. When the rock is initially harvested, it will contain critters that will die off and make ammonia, however, over time there is no more food for the bacteria and it also dies. It is a difficult balance for the store keeper because he/she can't put new LR in with saleable fish but can afford the room to keep the rock cycled.

When you take your water into a pet store to see if your tank is cycled, almost never do they ask if you have seen a ammonia and nitrite spikes. What results is people putting dead rock in their tanks, letting it sit for a month and then getting the OK to add critters because there is no ammonia or nitrite in the water. Unfortunately, there is often no bacteria either and the infamous, New Tank Syndrome occurs. If you Google New Tank Syndrome you will see tons of articles. Here is one of the first I noticed .

Enter an octopus. Octopuses are very messy eaters and produce more ammonia than a large fish. So even if you have actively (ie continually added ammonia producing cyclers) cycled your tank, observed an ammonia spike, a nitrite spike and then see them go to zero (usually about 1 month), you still need to continually build the bacteria to take the waste load of a large animal. This is why I try to stress the word active when talking about a 3 month minimum cycle.

So, what to do. Watch for heavy breathing. If you see it, change water ASAP. If you don't have a way to test your own water, invest in either reagents and/or test strips. I have come to rely on test strips because they are so easy and take only a few seconds but others frown on them (they are invaluable for acclimation). They do need to be fresh and kept completely away from moisture. Keep an eye out for ANY ammonia or nitrites (I suspect you may only see the later at this point). Over time your live rock will grow more bacteria. You can add more live rock but do take the precautions I mentioned by keeping it in a separate container with moving water for a week (or until you see no ammonia or nitrites whichever is longer) before adding it to the display. In an emergency, the chemical neutralizers have been helpful (do not add them to your new LR bucket!). I tend to shy away from them but none of my tanks are less than 5 years old or over crowded so I have not had to worry about ammonia for a long time. @sirreal recently faced an issue and adding a neutralizer may have saved his animal.
@DWhatley Ok so yesterday I certainly noticed the heavy breathing...
Next questions;
1- how much of a water change should I do? (gallons today, and what other changes this week) tank is 65 gallons including sump
2- there is tons of "live rock" at the fish store, obviously being new I chose the more visually pleasing ones as the shape had more character etc. There is however lots of live rock that is in the sump of the tanks there that have been there for ages, should I attempt to buy this stuff? Should I still cycle it for a week in a bucket or should I in an emergency put it in the tank or sump? They will give me water from their own sump with the rock so that shouldn't be hard to test for these levels?
3-what neutralizers do you suggest? Like you all say on this forum pet store owners know very little about octos and they now admit it when I ask questions. I'm very scared for my animal, we all love him so I will basically do anything to keep him alive and happy. Please let me know what you would do in my situation. I'm heading into the shop in an hour I will test and post my water results. Thanks for all your time
Good to see your on top of this and asking questions. As far as what to use right now there is a product called Prime from amquel. It detoxifies ammonia and nitrite so these things will not hurt your octo. IF you read the bottle it says in an emergence you can use up to 5x normal dose to detoxify. Most people in Fresh water hobby use a water conditioner to remove chlorine and other such items in our drinking water. This product does that also. Most of the water conditioners don't detoxify nitrites and as far as I know Prime is the only one I know of. Most LFS will have it.
Ok so here are my options for chemical treatments at the local fish store;
Suporia pad (removes nitrates and nitrites)
Prime by sequel
Also I did a water change (10gallons) the other day maybe 3 days ago. Should another be done already? Thanks for all the help
At this point I think I would do daily water changes of 2-3 gallons. The problem with large water changes, much like acclimation, is that they will change the salinity and, more importantly the ph where smaller, regular changes will be less stressful to the octopus.

What did your water test show? If you are not seeing ammonia or nitrites, I would not use the Amquel/Prime (similar products). They are supposed to be invert safe but I worry about them coating the gills. However, nitrite is deadly and requires less cautionary measures.

Are you using RO/DI water to make your saltwater or are you using tap water? If tap, switch to RO/DI if you can get it. If not use distilled.

Also think about your air exchange surface. If you are not getting good CO2/Air exchange, try adding an air pump and an airstone or two to the sump (not the display).

Yes, I would increase your live rock but, as I mentioned, just because rock has been in saltwater for a long time, that does not mean it contains much bacteria. Do they have rock that they will sell you that is in a tank with fish? I would put new rock in a bucket of circulating water in either case though. You should get negative results in a couple of days but I am more comfortable with waiting a week. What you don't want is to add nitrite.
Yes I'm using r/o water from the fish store, they have live rock that's in the sump of their fish tank so I will buy some of that and cycle it for a week or so in a bucket and do a test. I did a 5 gallon change since I'm due to change it tomorrow anyway. I'll keep up with the 2-3 gallons per day until my test shows no nitrites. The tank doesn't show any ammonia, so I think I'm good there. I have ammo lock on hand so I can use some of that if needed. The octo seems happy, swimming around and eating normal, there is no more heavy breathing, so I'll continue on this and hope for good results. I'll post more once I see a positive change in the nitrite level. Thanks again for the help
Is it possible that the test kit is not reading right? I have seen test kits show the wrong results many times. Are they your test kits of the LFS? If its yours have the LFS test also. You might not have an issue. D is right many small WCs are better then big ones.
I didnt think about the Prime coating the gills. Hmmmm. I am not a fan of any chemicals for a tank only in emergancy What are your Nitrites at now?
My nitrates are at 0.2 ppm I've tested them a ton and I'm using the Red Sea test kit.
I haven't used any chemicals yet. I did I 5 gallon two days ago and I think I'll do another 5 today if you think that will be ok
Wait!!!! do you mean nitrates or nitrites? I may be confused with another member and if I am I am sorry. If your nitrites are 0 and your nitrates are .2 your in perfect shape. If that's the case you doing perfect. Because octos are messy eaters its very hard to keep 0 nitrates. To keep from confusing nitrates and nitrites most reef hobby people use N03 for nitrates and N02 for Nitrites. "N03 is fine--- N02 is not" I hope this helps.

Shop Amazon

Shop Amazon
Shop Amazon; support TONMO!
Shop Amazon
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.