my rockscaping / question on cycling

I prefer to use liquid ammonia to cycle a tank (you have to buy janitorial strength now to get away from any harmful additives collectively labeled surfactants) , but there's plenty of folks that still use a piece of shrimp. It should not take anywhere near 8 weeks to cycle though if you do add cured live rock (keep it as wet as possible during transport) and actual live sand (not bagged live sand), unless the piece of shrimp simply decays that slowly for you. After you add your ammonia source, can measure the ammonia level you're targeting, then end up with 0 ammonia, 0 nitrite, and an expected amount of nitrate, the tank is cycled. The bacteria that break down nitrate work very slowly... very, very slowly... to the point that I would not count on them for nitrate removal, particularly in a newly set up tank. The macroalgae in your refugium will do far more for you in terms of nutrient removal.

That said, the bacteria performing the nitrogen cycle for you can start to die off if left for extended periods of time without an ammonia source. However, I would not expect that to happen in any significant fashion over the course of a couple weeks, unless some drastic change happens to the tank (very large and rapid spike or drop in salinity, temperature, etc.) The bacteria are pretty resilient.

When it comes time to add your fish, do so in moderation. Please remember that the bacteria working the nitrogen cycle exist in an equilibrium of sorts and need time to adjust to changes in the bioload added to the tank. Even in an established tank, too many new additions can cause a mini-cycle while the bacteria population increases to handle the new bioload. Short version -- please don't add 5 or 10 fish to your 75g all at once. If you have a list of fish that you know you want, I'd suggest adding the most peaceful / least territorial fish first and save any territorial fish as later / last additions.

When stacking your rock, keep this in mind... Any digging goby can topple over your rockwork if it is not stacked to be very stable. Some people go as far as attaching their rocks together so they cannot be knocked over. Even larger snails can act like a bulldozer and knock over smaller pieces of rock that may be near the top of your rockwork. Urchins are another critter to keep in mind in that regard as well.

Yes, the live rock you add will have all of the types of bacteria you need to seed your tank to run the nitrogen cycle.

In the vast majority of tanks, the bacteria that break down nitrate will never be present in large enough numbers to eliminate nitrates. Conditions in our tanks are not favorable for them to populate that heavily unless you get into something like vodka dosing or using bio-pellets. Your refugium will always be a major player in nutrient removal.

If it was me, I would not add any macroalgae to your refugium until AFTER you tank cycles, as the macroalgae can more easily use the ammonia than it can nitrate. You don't want the macroalgae to out compete the bacteria for the ammonia, which could really limit the bacteria's ability to populate your tank (and give a false indication that your tank has cycled). I'd let the bacteria do their thing, then once you only have nitrate, add the macroalgae.

It's not so much that the macroalgae consumes ammonia more quickly than the bacteria will, just that the macroalgae can more easily consume ammonia than it can nitrate. In a cycling tank where there's ammonia and almost no nitrate, the macroalgae will happily use the ammonia, leaving less for the bacteria to use in becoming established in large enough numbers to support the nitrogen cycle in your tank. Once a tank is cycled, normal amounts of ammonia are consumed essentially as quickly as they are produced, with nitrate levels tending to persist, which is what the macroalgae will utilize most often.