HI,
Depending on whats raising your PH many times it could be something new in the tank, or possibly you have a large shell that was bleached . Fresh carbon can crash the PH occasionally if not rinsed I am not sure. almost all tanks lean toward acid due to bacterial action on waste. What usally happens in most tanks is the new owner over feeds and you have a bacteria bloom due to food and fish waste this will lead to a ammonia problem . This can suficate the fish by burning their Gills ,,SOmetimes the PH will crash during the set up. You then runout of the free calcium carbonate (a natural buffer found in hard water)in the water and your PH will crash to low. Most fish die from PH injury and it may take a few days but they are going to die even if you fix the PH.
Here's a simple fix . Got to your favorite pet store and ask them if they have any tank bacteria in a bottle, there are two companies that sell benaficial bacteria for fish tanks.. You only need this for a brand new fish tank..
Start with a couple fish and wait a week or two for the tank to activate the filtration. I always turn off filtration when I feed this prevents the filter getting some of the food. Only feed very small quanities of food never over feed.. When you do tank maint. only clean 1/2 of the filters pads never do them all on the same day,, when you replace filters they take a couple days to start growin the good bacteria, thats whats clearing the water not the filter. I have large fish in my big tanks and I also have smaller fish in the same tank the smaller danio fish ( ps giant and pearl danio's) are an excelent fish for a new keeper).the daino eat what ever makes it past the gills in my larger species of fish this prevents tiny pieces of food from fouling the filters. I rarely do tank maint I rinse the sump filter maybe once a week. the canisters are every two months
When I break down my 100 gal tanks I always save the dirty filter's and replace the pads and then rinse some of the the dirty filters into the clean water in the fish tank by squeezing some of the gray out of it. The good bacteria will smell like potting soil.. This is the good bacteria your water will haze up for a couple mins and then will almost always clear up..this re-charges or activates the new filters much quicker and prevents any addtional
delay in removing any of the biological waste in the water.
Squeeky CLean filters don't work as well as charged ones.
Many people have spent years learnig how to keep fish some have luck with one species only.
Most petstore fish can have parasites and have been stressed by shipping and temp changes. Many will die in the perfect tank.
I havent' added a fish to my 100 gal in a couple years because I don't want to risk adding Ick to the tank.. If your constantly adding fish you will have problems in that tank.
enough for one day.. hehe
doug
psps, i almost forgot never clean anything that goes in a fish tank with any type of soap, never rinse filter in a sink that may have soap residue, if you must clean algae off the tank cover or some of the plastic hardwear I usally us marine salt as an abrasive cleaner this willcut thru the algae and not poison your fish.and you can rinse off the salt with tap water.. Everything you use to clean, (buckets,cups whatever)scrapers, and them green pads canot come in contact with soap. I have tank hoses and a buckets and an ice chest that I keep in the garage for tank maint, and if any one uses one with soap they are gona buy me a new one... hehe doug good luck that learning curve can be painful..
...lmao...again..no sign off by Jeff 
Shane
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