Fish are not the best offering for both nutrition and potential problems. I found two studies done to see if octopuses (to be used as food) could be raised on inexpensive fish and both concluded that the octopuses did not grow well on fish as a main diet. If you feed (or try to, there is a good chance fish will be rejected, even the tank mates killed were often only partially consumed) them, do it sparingly. Any saltwater crab or other crustacean (fresh or frozen) that it will accept is fine. An occasional freshwater crayfish is likely to be a desired treat but the current thought is to only use them for treats. They won't survive long in saltwater but the ones I have fed never died of their own accord. Live shrimp can be hard to catch so watch to be sure it has eaten. They can be left in the tank for extra hunger days and hunting excersize but if they are used for a main food supply (like days when you may run out of crabs) be sure they are caught. Catch, kill and offer one if the animal misses a meal or two.
Feeding quantity is unestablished. Roy firmly believes that their lives can be somewhat extended with reduced feeding days (roughly every third day for O. bimaculoides - using this schedule one of his wards lived almost 3 years). We feed ours daily and don't see extended life span but they live the expected amount of time. With Roy's 3 day program, I believe he feeds multiple crabs during a feeding session, we feed one a day so the quantity is similar but not the schedule. In our experience, they don't seem to overfeed so when there is left over food in the tank, we reduce the quantity until all is consumed. In general, younger animals will eat more than mature ones and we have noticed that aged animals need softer food (they stop accepting or leave most of the offered table shrimp but eat crabs and krill easily).
In most octopuses we keep, it is very hard to see the specialized tip. It is not really an extra appendage, but a modified tip and a channel down the third arm to the right (clockwise looking down at the animal with the eyes at 11:00 and 2:00) . Additionally, some species have enlarged suckers on a couple front arms (arm number and number of arms varies but will be on the same left and right pairs). Typically sexually mature males will carry this arm curled most of the time.
Here is a thread with photos to help identifying a male. There is no external characteristic for identifying a female but before brooding the mantle grows quite rapidly.
The 70 will make a nice home for most of the species we keep. If you get (or already are) hooked on keeping these guys it should serve very nicely for either octopuses or cuttlefishes.