1- One way you may be able to encourage some more early morning or late night activity is to shift the light cycle of the animal slightly. This would require you to get a daytime light or adjust the light cycle on your red light. You can do this by having the red light turn off at night (maybe between midnight and 6am), leaving it set that way for a week or two, and then adjusting it by 15 or 30 minutes every day or two in order to shift their "night time" to be earlier in the evening. After a week or two of adjustment their red light could be off between 8pm and 2am, which might allow you to see some more activity around 8-10pm.
Additionally, if you time your feeds you can start to shift your feeding schedule to the time that you want your animal to be most active in the tank. Start feeding at midnight and then as you shift the light timer also shift the feeding time to make it earlier in the day. After a few weeks you will be feeding around 6-8pm and your octopus will likely expect food at this time, especially if you are feeding every day or every other day. Ill touch on this point below.
As an aside- why do you have the red light running all night? That could be contributing to your increased water temp.
2- One crab every 3 days might be a bit light on feeds, but it really depends on how large the crab is and how active the animal is. With an octopus, you don't need to worry about over feeding it from an animal health perspective. They wont get fat and aren't susceptible to things like fatty liver disease. They can't overeat and will just leave food behind or ignore the crabs if they aren't hungry. You could try feeding every other day while paying close attention to how long the crabs stay alive in the tank. If you routinely see the crab be eaten soon after it was added to the tank, you could try to increase the feeding frequency further. I would suggest trying to feed every other day at first, and if you regularly see the crabs alive in the tank for long periods of time (if one is still alive the next day after feeding) then you know that you don't need to be feeding every other day and you can go back to 1 crab/3 days. The most important thing to keep in mind while increasing feeding frequency is to ensure that the octopus is actually eating the crabs you put in, and that the crabs aren't staying alive to bother the octopus. A more regular feeding schedule can also help you encourage your octopus to be active around specific times of day, when they expect to be fed.
3- I would recommend routine water changes every week, and maybe additional water changes on top of that if you start to see your nitrogen levels rising. 10% volume/week is pretty standard for most aquariums, but again- this depends on your animal, the water quality parameters, and how well established your system is. You check your water chems every week or every few days right? If you are going to try increasing the feeding frequency to encourage some more activity around specific times of day, you will need to also increase your water testing frequency. Make sure to remove the leftover crab shells soon after they are eaten (I wouldn't leave them longer than a day if possible). They can wreak havoc on your water quality parameters. Crabs are nasty.
4- what temperature would you like your tank at? I can make a few suggestions. What about the filter do you think heats up the tank water? Is there a heater inside the filter unit that you can control?
Lastly, when it comes to toys in the tank- some octopuses (especially young ones or shy ones) just aren't that interested in playing with toys. There aren't any pool toys or rubix cubes in their natural habitat, and often the best way to give your animal some enrichment is to swap up the aqua scaping in the tank. This will give your animal new places to explore and find dens to build- and you will often get much more engagement out of swapping the rocks around than you will by adding a jar or a kids pool toy. Just be sure to move slowly and not spook your animal when you are working in the tank!
Thank you SO much for your detailed reply.
In answer to your questions: I have the red light running 24/7 on the advice of another Octopus keeper. She said the 24/7 light means the octopus acclimates to the tank never being totally dark and it's easier to see them as they will come out more. Since Inky only comes out after 2am and it's red in the tank at that time, the pictures and videos are not the greatest. Which brings me to your suggestions of changing the light cycle. The tank sits on my kitchen internal window which means there is always activity and light from the kitchen. I have blocked the light from the living room side. I'll post pictures to show you. And it sits next to a freshwater tank which spills light into the tank during it's light cycle.
If I want to make it completely dark in the tank earlier in the evening so Inky comes out say at 8pm - I really can't due to where the tank is - it gets the kitchen light although I'm doing my best to live like a vampire - the fridge opens, the microwave has a light, the freshwater tank, the TV - all that light will still be coming in during this 8 pm Octo-night time. I've been toying with the idea of moving the tank to my bedroom, I only turn on my bedroom lights when I am going to bed so maybe that will work for him. I really don't want a tank in my bedroom though so what if anything could I do for the light cycle where it is currently? And should I not have the red light on at all then?
Inky has been coming out at 2-3 am with the light. He's not really moving around he sits on top of the barnacle or the rock and that's about it. I have started feeding the crab at 3 am. Bottom line is this dude ever going to come out when it's halfway daylight in the tank? Probably not.
Speaking of crabs I have all sizes and I've been feeding the smallest ones first. They're not that big and he does eat all of it and throw the shells at me. I have been removing them. The crabs don't live longer than a few hours so he is hungry, I have started feeding every other night as you suggested.
My nitrates have been high, I did a large water change yesterday and added a nitrate/phosphate pad for the filter and will keep an eye on it. I have not been testing but I will.The tank was set up brand new for Inky but with established live rock from my reef and I have been adding live nitrifying bacteria for further rapid establishing of bacteria. There is no ammonia, never has been.
The tank does not have a heater, it is running at 79 just with the filter and the lid closed. I have asked a friend to make me an acrylic lid with small holes so hopefully that will help. I put a vornado fan on the tank a couple of days ago and that brought the temp down a couple of degrees to a better range. My apartment is cold, even my floofy cat is cold! The things you do for an Octopus.
I had been re-arranging the rocks a couple of times a week already just to give him something to think about. Added a barnacle and he went in it very quickly. He's so interesting. I caught a video of him cleaning himself, it's like a laundromat with his tentacles rolling on themselves! I'll try to post it.
