with your salinity you don't want it to change so much so rapidly. You mentioned that it was at 1.024 when you checked the water parmeters, then went up to 1.027 that night, and is now down to 1.022 after topoff. These swings will cause alot undue stress on your tank inhabitants.

Is there any kind of 'rule of thumb' for adjusting the HG level when you are trying to make small changes?

I vote against adding anything to cause a spike in your tank. IF you've bought live rock from another source, then it's teeming with all the beneficial bacteria from that tank. Why spike your parameters and kill it off?

If you can... get rid of the fluval filter... its a disaster in a making... unless you are religious in cleaning that filter like every 5-7 days... and rinse it good.... and lots of water changes

In saltwater systems no mechanical filters are needed.... you rely on your LR and sand and water changes (Once a week and 20%)... seems like you are on a good start on that..you got plenty of LR and sand...

I would instead of using the fluval....add a HOB refugium. They are very popular with Nano systems that are not big enough to require a sump. In the refugium add some sand and cheato and you got a Nitrate Reducing Factory....

Seaclone 100 will be fine for now, but later on depending on your bioload it won't do much.... Remember protein skimmer, lights, and LR are where your budget goes to...

As for your lights... its fine for a FOWLR setup...if you plan on anything with corals...you need to switch your lights....

You will need more water flow as well.... For a tank your size....depending if you want corals or not.... with corals you should be aiming at least 1000-1500 gph total in your tank for soft corals... and even more for SPS corals..

Anyways good start for now...and I am happy to hear you got lots of LR as lots of people cheap out on the rocks...and pay for it later on :(

I used a funnel fron Canadian tire and a unused valve from a circ pump for a dripper. I hang it in the back of my biocube and drip in my chemicals/water top-ups. Works great for under $10. Also use it for aclimatizing new stock.

As far as the cycling is concerned. I had a similar situation with transfered live rock, and I didn't get a detectable cycle. You still need to wait a while before adding fish to be safe though....

The 'rule of thumb' is to not ever have to adjust your Specific Gravity. It should stay very VERY close to where you want it... large changes like what you've described above would kill or at the very least heavily stress many of the less hardy types of creatures you will likely want to keep. The ocean is a very stable environment and doesn't change that much, which is what you should strive for... stability.

And I agree with MCC about the fluval. I would empty it and use it to circulate the water, nothing more. There really is no need for a filter like this in tropical saltwater tanks, that's what your rock and sand is for.