Bio-Spira in my experiences DOES NOT work overnight... at least for Brackish of 1.004SG.
I used the Freshwater Bio-spira in my 40G(w/20G of water) with 4 Archerfish and probably about 75% of the bacteria died causing an ammonia spike of 8.0ppm. The next day I went out and bought 3oz of Saltwater Bio-spira... that batch survived and finished my cycle in about 1.5weeks.
This may have worked better if I had straight fresh or salt and not in between. For my second brackish tank I used Seeded Bio-Media from my 40G and it cycled in 7 days.
I'm not covinced that Bio-spira wont work. I just think you need to bring a cooler with you to keep the bio-spira cold (BUT NOT TOO COLD!), and use it in either full fresh or full salt with a full sized bio-load of fish so that the bacteria will have something to eat. It will take at least 24hrs... this is what Marineland claims so don't expect anything instant. Also keep your temp steady. If you're using delicate and/or expencive fish/inverts don't use it. My archers only survived because they're freakin' invincible.
GOOD LUCK!!