x2 on additives no need in small tank.
Kent Marine Liquid Reactor Additive
one part Liquid Reactor is a complete calcium buffer system
http://www.jlaquatics.com/product/k-...+-+16+oz..html
I would start at half dose and test calcium and alkalinity
General rule if you don't test it do
NOT add it to the tank you will make it worse, just do your water changes.