I am a major opponent of using fish to cycle an octopus tank. Many object because often the fish die in the cycling process but my objections are outside of any cruelty issue. Any fish hardy enough to aid in cycling will be an aggressive meat eater. They are very difficult to catch and you will end up disrupting your tank trying to do so. Because of the extreme difficulty and disruption, many people decide to "just leave it in there for food". This leaves a territorial, aggressive animal in the tank and no good ever comes of it. Sometimes the octo will be large enough and fast enough to kill the annoyance (sometimes eating part of it, sometimes not) but it is a stressor for the animal and will be a negative and unhealthy experience. If the fish survives, it is almost guaranteed to pick on the octo and can cause skin damage (eyes are always a concern here but we have not seen them blinded) that can lead to infection.
There is an adage that enthusiasts learn to be true, "Nothing good EVER happens quickly in a saltwater aquarium". We try to encourage you NOT to learn the hard way and emphasize a slow, and thorough cycle. You can't "see" that a tank is cycled and you will have to accept that it is a slow process to build up the beneficial bacteria that will convert the excessive waste from ammonia (deadly) -> nitrite (also deadly) -> nitrate (benign in small quantities). The biological process the bacteria provides will keep your tank from killing the animals. Water changes, good cleaning habits and good water flow will help remove the final stage of pollution. To build up enough waste, you need to feed the tank and have the waste go through the conversion process.
Porous rocks are a preferred substrate for housing and growing the bacteria. The introduction of live rock (rock that already has bacteria growing) is favored by many and will speed up the process. Artificial addition of bacteria (if what you buy is actually alive, often not the case) will only temporarily increase the desired level but if the tank is not well established, it will simply die off. Cycling a tank requires feeding it to slowly build the beneficial bacteria to stable levels. Another heavily experienced problem is commonly expressed as, "new tank syndrome" (look up both terms, you will find them prevalent through out the forums). This is a result of overloading a new tank with animals before there is enough stable bacteria to quickly convert the waste. You should actively feed a tank for a minimum of 3 months before thinking about adding an octopus. Octopuses produce a lot of waste and an uncycled or minimally cycled tank will become poisonous quickly.