Most of us use Prime because it's an excellent declorinator and ammoni-locker. Detoxifying ammonia is important in a small and/or cycling tank. It's a good value, because it's concentrated. It's also easy to dose: by the drop.
The pH is probably not critical but, if you're cycling that 2.5g tank, you really ought to have a liquid test kit for safety. This is what most of us use:
[ame=http://www.amazon.com/API-Freshwater-Master-Test-Kit/dp/B000255NCI/ref=sr_1_1?ie=UTF8&qid=1379756505&sr=8-1&keywords=API+freshwater]Amazon.com : API Freshwater Master Test Kit : Aquarium Test Kits : Pet Supplies[/ame]
A 2.5g tank is right on the cusp. You can cycle it or you can keep it unfiltered with just 2-50% weekly water changes with Prime (and an occasional thorough cleaning.) Your choice.
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