Can you take some of the filter media from the bigger tank and add it to this hospital tank? Technically, a 2.5 gallon is very hard to cycle and keep cycled... and pristine water is especially necessary when treating illness. The other option, which is probably safer, is to do 100% waterchanges in this 2.5g and add an airstone to the tank for oxygenation. Normally, in a 2.5 g... you can do once weekly waterchanges but if you're medicating with RI+, you'll need to do daily waterchanges prior to each dose.

As well... make sure you dose at the same time each day, and do the water changes just prior to when you're dosing so you're not removing the meds too soon from the water.

Michele