What?? 18000K for 20W? never see this kind of tube or bulb before. May be it is good for marine.

like you said 2)get some sodium bicarbonate...yup baking powder...3 tablespoon to be exact....and put in the tank for 2 days...stop all fileration. after that a 100% water change...add in some PSB or whatever bacteria and let the fileration run for a week....cover the tank with newspaper ...try to keep it really dark.

3 tablespoons of soda powder for 2ft tank might increase the KH from 0 to 2 immediately. Beside those tough fishes. Do you think other smaller fishes like tetra and shrimp able to stand of KH swing in suddent?

And stop all the filteration. No O2 provided? Fishes able survive for 2 days since non photoperiod and no O2 provided.