I top off with a kalk drip that I made myself. It cost me every bit of $14(US). All I used was a 5 gallon jug and a drip line purchased from a pharmacy.
Kalkwasser (German for literally Calcium water) replenishes Calcium levels daily and maintains alkalinity. Prepare the solution as stated on the bottle, then drip slowly into the tank. No too fast, as it quickly raises the pH if done so. A pH monitor is very helpful here.
Do you have a calcium reactor? If not, use kalk water with DI water for dosing. Is the tank acrylic? If so drill it to install a float switch (Kent makes one for $17.00) and gravity feed from a 5 gal jug. Another alternative would be to dose with a digital dosing pump. Just fill a 1000cc bag with solution, load the tubing through the pump, set the rate, hang the tubing over the tank and push go. You can acurately and slowly infuse anything! If you need a pump and bags sets, I'll sell you one. I have several, but please contact me for info since this is not the forum for sales. [email protected]
Cheers,
Scott