Originally Posted by ollief9
Well as 2.5 gallons is too small to really establish a stable cycle, you'd be better off doing 100% water changes once a month or so.
BS. Its not the size of the tank, its the kind of hardware used in it. I've got a Tom's small sponge that I cut down to a 3/4" ring around the base and it can hold a pop bottle stable. The most common cause of the whole "too small to cycle" misconception is in tanks that are overstocked, overfed or too cold; you can cause an ammonia spike that triggers then exceeds the level of the bacteria's dormancy response. The low temperature of a goldfish tank is the primary cause of the water quality issues in them.
If biological processing was an afterthought or just something the manufacturer "hoped" the device would do, it won't happen. Easiest way to make it happen is wedge a nice sponge into the filter.
Honestly I do 75% water changes each week after my filters establish biological capacity. I always siphon to some extent and I usually disturb the bottom before removing water. Waving the siphon head around low before and while siphoning really fluffs all the lightly stuck stuff up.