Just heard back from "Nora" at Hagen in New York, who called corporate in Montreal and was told:
Use 5 ml Fluval Shrimp Mineral Supplement per TWENTY gallons of water to increase 0.6 units dGH (German degrees hardness).
This should mean that
- one ml added to 4 gallons, or
- half ml added to 2 gallons, or
- 0.25 ml added per gallon
will increase the hardness of THAT water by 0.6 dGH
- These are given for those doing water changes, to calculate how much to add to your water-change water.
Don: Since this tank houses BTs and crystals, a good goal is middle of the road 4-5 dGH in the tank as a whole. Therefore, the best strategy is to make water change water in the correct ending concentration, and do a bunch of water changes until the tank as a whole is correct. For example,
Make 2 gallons of water change water at 4-5 dGH, which would be
half ml into 2 gallons gives 0.6 dGH. We need 4-5 dGH, which is (4.5/0.6=7.5 times this much, so 7.5 times half ml = 3.75 ml)
3.75 ml Shrimp Mineral Supplement into two gallons new RO water - this should make 4-5 dGH water in that two gallons - can someone verify this with a test kit, please - not the calculations but the actual result, as I am not 100% trusting what they quoted me - it makes no sense for them to bundle a product for an 8 gallon tank and give instructions for the product for twenty gallons..
Take the two gallons and do a drip-in water change (remove 2 gallons old tank water, then drip in the new water) ONCE A DAY for about two weeks, making a new 2 gallons of water change water with this recipe each day.
DO NOT ADD ANY FLUVAL SHRIMP MINERAL SUPPLMENT DIRECTLY TO THE TANK, BUT RATHER ONLY
TO THE WATER CHANGE WATER, using this method.
Over the two weeks, the TANK water will gradually increase to the final 4-5 dGH value if you do this every day for two weeks.
Thereafter, you do a water change using 2 gallons of this same water mix, weekly or more often. Keep the tank topped off with DI water in between (mark your tank water level with a mark or piece of tape, and add DI water to this level every night if the level has dropped due to evaporation), so evaporation in such a small tank doesn't make the water concentrate over time.
This is about 25% water change in this tank, if you consider some of the 8 gallon volume is displaced by substrate, etc., - a little higher percentage than would be done in a large tank, but the slightly higher ratio also helps keep your tank water from creeping up over time. Small tanks have to be maintained slightly differently, due to the evaporation risk.