Question:
A few years back I was experimenting with a homemade filter system based on the type of canisters you get at discount stores, which are used in home water purification systems

Something added to the cartridge that is not present in specially made aquarium cartridges, which makes it particularly suitable for purifying drinking water? I thought so at first, BUT...

I have since heard of other incidents of this nature involving aquarium carbon. The onset of the cloudiness seems to occur 2-3 days after changing the carbon. I have also heard (but never been able to verify) that new carbon can cause pH shock.

What is the group's experience with the former, and is there any (verified) truth to the latter?

Answer: If I had to take an educated guess, probably what happened is that activated carbon for fish tank use and activated carbon for house/drinking water use have one key difference. In those drinking water or whole house carbon inserts, many of them use, ummm I think some sort of silver additive, as an antibacterial. After all when you have an under the sink carbon filter and it sits for a day or two without use, you don't want bacteria growing in there so your next glass of water comes out smelling awful, let alone actually tasting it. Now using these under you sink and getting water from them and then putting it in your tank if perfectly fine since the "silver" remains in the filter under the sink and not in your tank.

However, when you come up with a DIY filtering system using these parts, and specifically these carbon products, that is going to be run directly on the tank, well then I am guessing what you see happen is a HUGE reduction of all the bacteria in the tank and effectively the destruction of your biofiltering. So now you suddenly have this un-cycled tank with a full fish load. So the ammonia levels immediately start to skyrocket and of course fish start dying.

As many of us recall from our first tanks and as we see posted to these groups over and over again, one of the first "problems" with a new tank is the notorious "cloudy water" syndrome which is simply a bacterial bloom trying to catch up with the ammonia levels. So I would guess that the cloudiness you see 3 days or so after trying one of these "house" filter cartridges is your tank trying to recycle.