There is a method of cycling FW called the planted cycle, which is very similar to cycling with live rock. Basically you stuff the tank to the max with healthy plants and add a fish, then work slowly on stocking from there, the plants will absorb some of the ammonia and also help seed the tank with their bacterial colonies. It's not as sure fire as LR, and might be really difficult in a 2.5, but is possible, especially if you start off with some shrimp and don't plan on over stocking anyway. I would just try to get all of the substrate, filter if you're going to use one, and plants from a well established tank.
I don't have any links on hand, but the FW nitrifyers take about 2 weeks for the ammonia eaters to populate fully, then roughly another two weeks for the nitrite eaters to fully populate, that's without any help from established media or plants. So, any nitrogen cycle graph will show you a month long time line with ammonia rising from day one (after stock is added), spiking through the first week, dropping the second week while nitrite spikes, then finally a nitrite fall and nitrate buildup the last week. I managed to get my last one down to 6 days with plants and by running it's filter on an established tank for a month before setting everything up, I also had the help of 3ppm ammonia in my tap water, so I didn't have to add fish right away, I watched the 3ppm turn to 0 then added a fish and watched nitrites disappear by the next day. I basically used the filter as the LR, but with LR you have die off that starts the cycle, where as in FW you need to add something to kick it in gear. Some people use household ammonia to do a fishless cycle, but it's not that complicated when you stuff the tank rim to rim with fast growing plants.
There's also the option of Bio-Spira from Marineland, if you don't feel like stressing it and have the money. It's the only quick start that actually works, it has to remain refrigerated because the nitrifyers are in suspended animation so to speak, ready to go as soon as they hit the water. You add dechlor, wait an hour, add biospira and fish at the same time, the cycle will be done in anywhere from a day to a week as long as the culture didn't thaw and starve in transit.
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