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Water Changes and Ion exchange Resin chemistry (Algae)

    The idea behind water changes, is that you are replacing missing ions, 
and removing unwanted ones. But this is only true if the water you are 
replacign it with is good quality water. And I believe that BBA a red algae 
is due to high phosphates? Try some Duckweed, it's a personal favorite in 
limiting phosphates, as I can't afford to add any more bioload with algae 
eating fish as it is. (no, none of my algae eater prefer algae to live worms 
:-P )
    I do 20 gallon water changes on my 75 gallon tank (actually only has 
maybe 50 gallons max of water, the rest is filled with substrate, plants, 
wood, fish etc...) every two to three days, and I think that is the upper 
limit on water changes for a hobbyist. But I only change the water as I see 
necessary to keep algae at bay. I use RO/DI water remade with Calcium, 
magnesium and trace elements in a commercial mix. However, as far as fish go, 
all you need is Calcium and Magnesium, as they do not absorb/adsorb any 
nutrients through the water (except oxygen, and water). Only plants 
absorb/adsorb nutrients through the water. Fish on the other hand must be fed 
their vitamins. But I think my comment is argueable, as Many Discus raised in 
too soft water get what is commonly known as "punk Discus" that is their 
dorsal spines aren't fully connected, this is due to being raised in too soft 
water, and/or not getting enough vitamins when they were being  raised, and 
there is NOTHING you can do with fish after they are 1.5" big, if they have 
spikes at that size, they will ALWAYS have their spikes... :-(
      Some Discus Breeders do 50% water changes daily, but that is usually 
only the circumstances when they have nearly perfect water coming out of the 
tap (i.e. southeast Asia where no chlorine, soft water etc...) Some breeders 
run an open system, that is constantly running new water through their 
systems. On the opposite side, some people never change their water. 

Water Change alternatives: Ion Exchange Resins 
    If you are able to test for all elements which would be harmful to your 
fish and plants, then there is no need to change water, merely only add or 
take out what is necessary for their health. On the other hand, if you don't 
have the tools to measure anything and everything that can be toxic or 
harmful to your plants and fish (I believe that only one of us with a mass 
spectrometer does :-P ) then it is necessary to make a bet, that at a certain 
point, some ions will be out of proportion. 

     Another option is Ion Exchanger resins (H and OH, not to be confused 
with cheaper Sodium exchanger resins) which some reef aquarists do more than 
freshwater people do  (because Reef tanks have plenty of Alkalinity to 
spare). They  run a Cation exchange unit (not a mixed Bed!) through their 
tank. Remeber that Cations are the negatively charge ions like PO4, CO3, SO4, 
anything that goes second in a chemical formula (ie. the SO4 sulfate ion in = 
magnesium sulfate MgSO4) The problem with this, is that before you ever get 
to taking out the bad ions, you will remove all of the buffering capacity of 
your tank, which would drop the pH to 1 in relatively short amount of time. 
The other problem, is that you do not eliminate the toxic Anions (Mg, Fe, Pb, 
Fl, Xe :-P for those of you who got that little chemistry joke :-P ) that may 
be in great quantities.
     To further discredit the idea of simply say, running a "Tap Water 
Purifier" cartidge (a H+ and OH- mixed bed Ion exchange resin also called a 
"strong base and strong acid" ion exchanger, as opposed to a weak ion 
exchanger resins which in genetics they use for removing certain long protein 
chains I belive) through your system, is that the process of taking certain 
ions is not a selective process (except for some heavy metal resins which are 
absurdly expensive, and not meant for hobbyist use, only for large scale 
water treatment? right Wright? or who ever is a certified water treatment 
specialist?). This means that the resin will take out everything that is more 
postively or negatively charged first, and take less electronegatively (or 
postively?) charge last... And guess what? that's right, our problem ions 
PO4, NO3. or most commonly problematic ions are the last ions to be picked 
for the Resin Ball team :-)

    Anyone know of a good source for H+ Anion and OH- Cation exchange resins? 
Anyone done a DIY Ion exchange resin system? I'd be interested in hearing. 
    All chemists, feel free to thrash my beguiled General understand of 
Chemistry, though I think I got the general idea down. Hope it helped, Best,