Hav­ing a strange prob­lem with rain water…

So, I col­lected a large quan­tity of rain­wa­ter recently, around 90 – 100 gal­lons (about 300 liters).

I nor­mally do not have a prob­lem with rain­wa­ter. I col­lect it from my roof and then run it through fil­ter floss/​physical fil­tra­tion media to remove small pieces of organic debris. Then I usu­ally run power fil­ters with acti­vated car­bon to remove any poten­tial chem­i­cal con­t­a­m­i­nants, and store it for use later.

I was test­ing the pH of the water I col­lected out of benign curios­ity, and found that to my dis­may, the water that had not yet been fil­tered chem­i­cally had a pH 0f 6.4 while the water I was fil­ter­ing with the car­bon now had a pH of well above 7 (it seemed to be around 7.5.

I never had this prob­lem when I was col­lect­ing it in smaller quan­ti­ties with plas­tic con­tain­ers lay­ing on the ground, but for some rea­son run­ning the car­bon raised the pH. I don’t under­stand why this could be hap­pen­ing, and after toss­ing the rain­wa­ter with the increased pH, using new acti­vated car­bon and rins­ing the fil­ters with dilute phos­phoric acid (and rins­ing again with reverse-​osmosis water) and even rins­ing the acti­vated car­bon with more reverse-​osmosis water, it hap­pened again, and the liq­uid pH test came out to approx­i­mately the same value.

I also ran repli­cate tests the next day and the same results appeared. I don’t want to have to dump all that rain­wa­ter, but I also feel a lit­tle ner­vous about using it straight from the bar­rels. I don’t live in an area with much heavy indus­try, but I’m not one to make assump­tions about things like this.

Could it be the car­bon doing some­thing to alter the pH? It seems far-​fetched to me, since if that’s what is hap­pen­ing it means that the car­bon really isn’t suit­able for aquaria that are meant to have more acidic water, but I’ve never had any trou­ble with the brand before.

Does any­one have any ideas? I’d rather not throw all this free water away…

In water with­out car­bon­ate hard­ness (rain­wa­ter) of PH is very labile.
Small­est amounts of acidic or alka­line sub­stances cause quite a change.
How high is the elec­tri­cal con­duc­tiv­ity? pre– u. after fil­ter­ing?
So slight vari­a­tions are usu­ally harm­less.
PH mea­sure­ments in water with low con­duc­tiv­ity are any­way dif­fi­cult -
and usu­ally wrong.

Well, I think I may have answered my own ques­tion.

After search­ing for other instances of this prob­lem, numer­ous threads and online con­ver­sa­tions came up; other peo­ple have had prob­lems with this as well, and it is indeed linked to the acti­vated car­bon.

Finally, one thread on a marine fish­keep­ing forum detailed that acti­vated car­bon can raise the pH of the water it is used on if it was not acid-​washed first. At some point dur­ing the pro­duc­tion process, ash is pro­duced or acquired as a con­t­a­m­i­nant. Some brands/​grades of acti­vated car­bon are treated with an acid wash, which dis­solves the ash (ash is often com­posed of metal oxides), but it seems that there was also some back­lash against this prac­tice, since it could alter the pH in the “wrong direc­tion” for cer­tain appli­ca­tions and since phos­phoric acid was often used, which meant that phos­phates were left in the car­bon and con­tributed to algae prob­lems in aquaria.

The only real solu­tion appears to be find­ing a brand of acti­vated car­bon that is acid-​washed, or to use an acid/​acidic solu­tion to wash/​rinse the car­bon with prior to using it on the rainwater.

I know this is old now, but I wanted to update it with some­thing that might be valu­able to other peo­ple.

First of all, I have since stopped using acti­vated car­bon in all of my tanks, paros and oth­er­wise. It’s too dif­fi­cult to find a brand that actu­ally will not raise pH and sponge fil­tra­tion seems to be work­ing bet­ter all around any­way.

Sec­ond, the rain­wa­ter was dirty for a few rea­sons. We had a very dry and hot sum­mer, which meant that more debris and air pol­lu­tion ended up stick­ing to the roof, and then the rain washed it down; I have since found that keep­ing the roof clean (or at least keep­ing the sur­faces and gut­ters that con­tribute to one’s col­lec­tion appa­ra­tus clean) by sim­ply rins­ing them with tap water from a gar­den hose (i.e. stand­ing on the roof and spray­ing it down) goes a long way towards keep the col­lected water clean. I usu­ally wait to set up the col­lec­tion appa­ra­tus until the rain has had a chance to rinse away any min­er­als from the tap, but this does not take long under sus­tained mod­er­ate to heavy rainfall.