do more frequent, but less % of water changes. 10% a week is fine. If it's a small tank, buy yourself some Seachem REEF salt. Your tank inhabitants don't use a lot od calcium or Alk from what you've list, and 10-15% water changes should keep it good enough.
here's some info:
http://web.archive.org/web/20021127040526/http://www.advancedaquarist.com/issues/nov2002/chem.htm">http://web.archive.org/web/20021127040526/http://www.advancedaquarist.com/issues/nov2002/chem.htm</a>
For systems with a pH of 8.2 or above, baking soda (sodium bicarbonate) is a good choice. For systems with a pH below 8.2, washing soda (sodium carbonate) is a good choice (though use some baking soda too if the correction is a large one and the pH gets too high; that is, above pH 8.5 or so).
<p style="text-align:left"><span style="font-family: Times New Roman"><span style="font-size: 13px">In gauging how much to add, here are some rough guidelines:</span></span>
<span style="font-size: 13px"><span style="font-family: Times New Roman">To raise 50 gallons of tank water by 1 meq/L will require 10 grams of washing soda (sodium carbonate). Since a level teaspoon of washing soda weighs just over 6 grams, then 1 teaspoon will raise the alkalinity in that 50 gallons by ~0.6 meq/L (~1.7 dKH).</span></span>
<span style="font-size: 13px"><span style="font-family: Times New Roman">One special note about washing soda: Apparently some Canadian brands of washing soda contain surfactants. VIP brand, in particular, contains them and a reef keeper using them on my advice turned his tank into a bubble bath. On questioning, the manufacturer did indicate that a surfactant is present. The same reef keeper says the local Arm & Hammer brand in Canada smells strongly of perfume. I’d avoid perfumed brands, if possible. My Arm & Hammer Super Washing Soda purchased in the US apparently contains no significant surfactants, and is not perfumed. Nevertheless, anyone using washing soda for the first time ought to put some in water and stir it around to see if soapy bubbles form. If so, I’d suggest finding another brand.</span></span>
<span style="font-size: 13px"><span style="font-family: Times New Roman">Many commercial alkalinity supplements will also be fine for this purpose, as long as no significant calcium is added. In general, I don’t prefer those that contain substantial borate. The alkalinity component of the two-part calcium and alkalinity additive systems would be OK. You [B]CANNOT[/B] use limewater or a calcium carbonate/carbon dioxide reactor to correct this problem. Any of the balanced calcium and alkalinity additive systems will move you parallel to the line at the left edge of the zone, while you want to move over to it, and cross it. </span></span>
<span style="font-size: 13px"><span style="font-family: Times New Roman">If alkalinity is more than 4 meq/L (11 dKH; an uncommon situation), then you can safely either do nothing until it drops and you need to add more alkalinity, and treat it as suggested in the previous two paragraphs, or you can add some alkalinity immediately, move into zone 1, and then just let it drop on its own</span></span>
