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When the quantity of life forms in the aquarium that require calcium rises, the demand for calcium in your tank will rise at the same time. Aged tanks and tanks with lots of corals will place a far greater demand on calcium levels than younger tanks and tanks with fewer corals. That should be pretty obvious.
Whereas at first this may not have a significant impact on the calcium levels, eventually calcium will fall and the amounts you add to replenish calcium concentrations will not longer suffice. What I am saying is that the demand for calcium in a tank is not "stable" or "constant", but fluctuates and usually rises. The more corals the more calcium is needed to provide those corals with the calcium they require and maintain calcium levels in the usually recommended ranges at the same time.
This is particularly so when coralline algae suddenly starts to grow all over the tank. Of course any other calcium requiring coral contributes to this phenomenon as well. Coralline algae may not grow right from the start but once the tank is a few months old and you have all the parameters adjusted, you may suddenly see a rather sizeable increase in their growth (this is especially so if the alkalinity is high or at least higher than what was recommended for some time).
Coralline algae appear to spread and grown more at dKH levels somewhat mroe elevated than the 6 or 7 dKH that was recommended for a long time when hobbyists used KW to increase or maintain calcium in the tank. The advent of two part calcium additives has made it possible to increase not only the calcium ppm level but also the dKH.
Hobbyists who have raised their calcium levels to 450+ and their dKH levels to 11 or 12 report a far greater growth of coralline algae than those who keep a lower dKH. This larger growth requires more calcium additions to maintain these corallines healthy and to sustsain their further expansion and growth. The more there are the more the calcium in the tank becomes depleted and the more rapidly this occurs.
This calls for more testing of calcium levels as the levels will fluctuate more.
Also, hobbyists will have to determine how much and what type of calcium additive to use to maintain the Ca++ levels at desired concentrations.
It should be quite obvious that when the calcium demanding bioload increases, the amount of calcium increasing compounds (whatever they may be) that were enough at one timet to replenish calcium levels, will no longer be sufficient and you will need to adjust the amounts added. If more animals and algae lower it by removing it from the water, the hobbyists will have to add more to make up for this depletion. What was once enough and maintained adequate levels may no longer be. As I indicated this is more so in aged tanks and in tanks with large amounts of corals and good coralline algae growth.
As the demand rises, the supply has to be increased.
This fact is often overlooked. Hobbyists who test "infrequently" then suddenly find one day that their calcium concentration levels are too low.
Or worse, they do not test, keep adding what was enough months earlier, and suddenly find that their corals start to look drab and may even show signs of detaching from their skeletons. In essence the calcium levels may have fallen so low that their levels cannot sustain the requirements of everything that is in the tank that requires calcium.
Watch out for this carefully and test you calcium levels regularly and this will not happen to you. Whenever you add animals or whenever you notice that the amount of coralline algae is increasing, you should test your calcium concentration levels more often, and you should adjust the amount of calcium additive you dispense to compensate for the higher calcium demand.
Where the problem usually starts is from the fact that most hobbyists use limewater to add calcium to their aquariums. They do so by replacing all evaporated water with kalkwasser (limewater). Since evaporation rates stay pretty constant, they are in reality adding the same amount of calcium all the time.
As the demand for calcium increases the amount added in the above fashion may not be enough and supplemental calcium addition may be necessary. Other calcium adding compounds can be used or the strength of the limewater can be increased by adding the milky solution rather than the clear liquid.
Since there are many other calcium supplements on the market you should have no problem supplementing the KW addition with another such supplement. Two part calcium additives are probably the way to go to increase the availability and raise the dKH at the same time. Several such products are now available on the market.
As long as you test regularly, you will know when to adjust the quantity that needs to be added to ensure that your levels stay at concentrations of around 450 ppm.
Just a useful tip that you may want to add to your list.