there is a larger coral frag farm that is exparamenting with using a second Ca reactor on there fresh water top off and they are having some good results from this but it is not enuf to keep up to the demands of a frag tank on its own. like I said it is in adition to a traditional Ca reactor. I was a little sceptical about it 3 months ago when i was talking about it and pounding out Ideas with the owner, but his latest updates are showing good results.
As Darren said they are not as efective in fresh water as in salt water but yes they will do a ok job of remeralizing fresh water and by doing this it will help the main Ca reactor cope with higher demands by giving it a head start with the make up water.
for the average hobiest this is not the path to follow and a traditional Ca reactor will probably cost about 100.00 including the pump to make your self. Don;t worry about weather it is up flow or down flow, or if you have 20 or 60 bubbles per minuit.. this is stuff to worry about when you have nothing else to worry about
I slapped togeather a quick one to get it going and then I started working on one of my own design. to be honest my quick one is doing a heck of a job, it runs at 20 bubbles per minuit and maintains my tank at 450 Ca and 12.6 ALK so in reality I don't need a bigger/better one but I want it
you probably could get what you are describing to work, but you would go throught 10 lbs of co2 in a week and for the reasons Darren mentions.. by closing it in you make it more efficient, and that is the whole idea behind adding a Ca reactor in t he first place... It is a more efficient method of maintaining Ca and Alk. and the traditionl type Ca reactors do this just fine.
Now if Darren ever puts up planes for his new fangled one so we can see how expensive it would be to build and after he does some long term tests in a loaded tank to determin its maximum capibilities, then it might be better to build his new design if it infact is cheeper to build, remeber there is nothing wrong with cheeper if it does the same job
My Ca reactor goes by two different principles which Darren should still be familier with "Brute force and ingnorance" and "go big or go home"
I am not worring about weather I get 10 bubbles per min or 100 bubbles per min.. although it would be nice if it is closer to the lower end and I am expecting about 20 bpm but only time will tell. the main drive behind mine was to be able to load it up with 16 lbs of media instead of 4 lbs and still fit it under my stand as I hate it when I have thins sitting out in the open.
Now to answer your question sence I just rambled on for a long time.. basicly you are using C02 to drop the PH of the water in the reactor.. you have a bigger pump (200 to 500 gph) to cirulate the slightly acidic water throught the media. this effect will give you a volume of water that becomes saturated with minerals.
now you also have to have some means of taking a smaller flow from the tank and add it to the reactor.. (I use a small powerhead for this) this is the amount that everyone refers to as a small flow (100ml/min ect.) by adding the tank water to the reactor it is mixed with the mass of recirulating water and very quickly becomes saturated also. Because you added water you will be also pushing the same amount out, which goes back to the tank, this saturated amount mixes with the tank water and adds Ca and other minerals to the tank water.
Steve