Maybe you need very little supplement. I'd suggest trying without it for a bit and watching the alkalinity.
You do not see undissolved material left in the 5 gallons after it has set for a bit?
FWIW,
Tropic Marin's directions say it should be added dry:
"Sprinkle Tropic Marin® BIO-CALCIUM ACTIF onto the surface of the water in a place with a strong current, preferably in the filter sump, making sure that no undissolved crystals come into contact with the creatures. If this is not possible, we recommend the use of BIO-CALCIUM ACTIF Liquid Set."
and I discuss it here (which will be published in an updated form in a couple of weeks):
Chemistry And The Aquarium: How To Select A Calcium And Alkalinity Supplementation Scheme ? Advanced Aquarist | Aquarist Magazine and Blog
from it:
One-part balanced additive systems: Salt Mixtures
Another type of balanced one part additive is comprised of a simple dry mixture of sodium bicarbonate (or carbonate) and calcium chloride. Just as with the two-part additives described below, this type of system can be further formulated to have a natural seawater residue after removal of calcium carbonate. Tropic Marin’s Biocalcium seems to fall into this category, though it’s written descriptions are notoriously difficult to interpret. It costs about $8 for 500 grams (estimated to contain about 1800 meq of alkalinity), so that puts the cost at about $4.40 per thousand meq of alkalinity. It claims to add 79 trace elements to the tank, along with the calcium and alkalinity, but doesn’t specify amounts for any of them.
You cannot mix this type of additive in water prior to adding it to a tank. If you do, the calcium will react with the carbonate present to form insoluble calcium carbonate. Consequently, the directions advise adding it directly to the tank. If you do, be sure to add it in a high flow area away from corals (like a sump), as the solids are reported to irritate corals if they land on them.
If you use a product like this, be sure to keep it as dry as possible, even to the extent of keeping it in a sealed container to keep out atmospheric moisture. If moisture enters the mixture, it may allow the formation of undesirable calcium carbonate.
Continual use of products like this will increase the salinity in the tank. The rise in salinity over time can be roughly calculated, though not knowing exactly what is in it makes the calculation only a ballpark figure. For every 1000 meq of alkalinity added in this fashion these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined below to be 18.3 thousand meq of alkalinity per year in a 100-gallon tank (50 meq/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S =35). In a high demand tank (defined below to be 219 thousand meq of alkalinity per year in a 100-gallon tank (600 meq/day), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using this type of additive, especially in a tank with high calcification rates.
