I'm not sure if he's eating his arms but it looks like it, and I bought the already cycled water is that bad? Also I just got him so could it just be stress that he's not eating? because he was eating at the pet store I gave him a cleaner shrimp and a small snail but it looks like the hermit crab got to the cleaner shrimp because it was already dead but I'm not sure. My mom checked the PH levels and salinity and those were fine, could it be the temperature? it's at room temperature
The water parameters that most commonly cause problems are related to the nitrogen cycle, so you'll need to test more than just the pH and temp to get much help here. Ammonia, nitrate, and nitrite are usually what people test. That also relates to the cycling issue, normally running those is the way to tell if your tank is cycled and stable before you get an octopus. The other test that's useful is for copper, because copper-based medications used on fish often stay in a tank pretty much forever, and they are toxic to octos.
I've never heard of buying "cycled water" before, but I doubt it's a substitute for cycling your tank properly. The cycling process isn't so much about the tank itself as it is about establishing appropriate colonies of bacteria in places like the live rock, substrate, and filter media, so although putting water from a cycled tank in might provide a some bacterial cultures as a starter, it wouldn't provide a stable system by itself.
I'm concerned that your tank is just not ready for your octopus, although it's hard to tell that without full test results, particularly on the nitrogen cycle. I'm trying to give a quick, emergency answer here, but the ceph care experts may be able to offer better suggestions.
Everything you report might be the result of ammonia not being handled because the tank isn't cycled yet, but the only way to tell that for certain is to get the test kits for ammonia, nitrite, and nitrate. If nothing else, doing those tests seems like the first thing to rule out.
If the octopus is in a tank that hasn't been cycled properly, and I'm pretty sure that "cycled water" isn't a substitute, then probably the ammonia, nitrite, and nitrate levels are way off and will be for a long time. Doing emergency water changes might be a quick fix to avoid killing the octopus immediately, but could also prolong the cycling time... unfortunately, during cycling, it's normal for the tank chemistry to go through phases that will be bad, and maybe fatal, to the octopus: that's why we recommend cycling tank completely before getting the octo. Hopefully one of the more experienced octo-keepers will chime in about whether an emergency water change is a good idea or not, since I'm not quite confident on that.
If I'm wrong, and the nitrogen parameters are not a problem, it's possible that the octopus is just stressed from travel. It's not at all unusual for an octopus that's just been moved into a new tank to want to hide and avoid coming out any time people are around, or lights are on, or it has any reason to be uncomfortable. However, you list a lot of things that the octo would do if it's very stressed from water problems. I worry that if you don't investigate and address the problems, your octo's in serious trouble.
Getting tests for ammonia, nitrate, and nitrite as soon as possible should be your first priority. If these show that this is the problem, do you have access to any other cycled tanks where you can move it until your tank is cycled? Returning it to the LFS might be a possibility, too.
oh, and to TONMO, sorry you didn't find us until there's already a crisis going on.
Thank you and yes my mom has a fish tank that weighs as much as a car do you think I could put the octopus in a container or something until then? or should I just put some water from my mom fish tank and put it in my tank? because the pet store owner said to do that. I put a towel over the tank so the octopus doesn't have to deal with me walking by my door every 5 minutes. I'll do some tests right away thank you
How long has the tank the octopus is in right now been up and running? If it's less than 2-3 months old you DEFINITELY need to move it or it is VERY likely it will die very soon, REGARDLESS of whatever advice your LFS employee told you. I know lots of live sand packaging advertises it as being a way to immediately cycle your tank, but this is false advertising. It will seed your tank with beneficial bacteria, but it will NOT cycle your tank.
If you can rule out autophagy (eating its arms) put the octopus in a plastic container, poke holes in the lid (enough for good water flow) and slowly acclimate it into your moms tank. That is, if your moms tank has been up and running for several months. If it is autophagy I don't think there is anything you can do about it, sadly.
If your octopus is a dwarf, like you think it may be, it is probably just wrapping its arms around its mantle, kinda looks like it's turning itself inside out. I HOPE that's all it is for you and your octopus' sake.