You would not be able to remove the mantis shrimp (at least not without disassembling your tank) so no, not a good idea. Mantis, like cephs, need a species only tank.
You won't be able to keep photosynthetic animals with 17 watts but you can look for some of the non-photosynthetic gorgonians and sponges (these are hard to keep alive but several require low light).
Yes, if you add fish, you would not keep an octopus. Understanding that you want to see movement would suggest that A. aculeatus is going to be your best bet for species but heed the warnings about what you order vs what you might get. Toys are usually ignored by the smaller animals but even with the GPO's (Giant Pacific - far too large for a home tank), keepers remove the toys after a short time. The news media tends to hype the toy part of stimulation but live food, and things it can safely watch (several observations suggest keeping a fish tank within view may keep them entertained without harm but it is not fully understood if this is because they fear predation or satisfies curiosity. I am in the later camp and my octos see each other as well as a fish or two - at a distance) are closer to reality. In his memories of long time cephalopod specials, Roland Anderson,
@gjbarord mentioned that Roland half jokingly suggested that perhaps putting a large predator in the tank would be the ultimate enrichment
. Keeping a nano with fish might serve to satisfy your desire to see ocean movement, keep a few corals AND provide enrichment.
One other alternate you might consider, now or in the future, is keeping cuttlefish. These are also personalitied cephalopods but the S. bandensis tend to swim/hover rather than crawl into a den. They have many of the color and skin changing attributes of an octopus but are more visible and move about more often. The biggest drawback is the cost of feeding them.
Your tank has reached the initial cycle but cycling is an on-going process of building bacteria (not stabization) to handle waste products. If you were building out a fish tank, your tank would be ready for a
small number of hearty fish that would eat food, poop and continue to grow the bacteria that will convert ammonia to nitrite and nitrite to nitrate. Overfeeding your cleanup crew does the same thing. An octopus is the equivalent of a very large fish or a tank full of smaller ones so a newly cycled tank does not have enough denitrifying bacteria to handle its waste. Growing algae has no benifit.
Maybe they are more intelligent... they don't escape from their tank...
