Not understanding pH in my tanks

[chris_topher]

So I know what pH is, and acidic and alkaline, how the scale works (0-14), things that raise and lower it, and that most freshwater fish prefer it a bit acidic or neutral. I also have heard that as the water gets older in a tank, the pH should slowly decline.

The thing is I just dont understand how this can happen: in both my 5g and 10g the pH is always high and today it tested 7.8. It seems to raise in pH as the water gets older. I dont have any crushed shells, I have natural gravel and i've tested my tap water (which is what I always do my water changes with) and it is 7.0 right out of the tap. My betta seems to be doing fine, but I plan on getting other fish now that my 10g is all set up, and I dont really want to kill them from high pH. What is making the pH raise and why is it not declining like it should?

Tank stats:
10g, 2x15W 6700K, Elite Hush 5, 1 Betta, 3 Amazon Swords
Ammonia:0
Nitrite:0
Nitrate:0
Temp: 26C
I dont have a test for hardness, I dont know if thats important or not, the water feels quite soft

 

[thincat]

First off, how often do you do water changes? Evaperation could cause the PH to raise up. Water hardness is the amount of minerals in the water,ie... Calcium, mg, salts etc. This is a hard one..no pun intended and I will keep tabs on this link.
Give us as much information as you can.

 

[chris_topher]

Water gets changed about every week and a half, sometimes sooner. I gravel vac too, then after filling I add a fair amount of reg flourish (half cap) and dechlorinator.

Edit: Just adding some info, I did see on my city's aquarium forum someone suggesting that the natural gravel where I got mine could have crushed shells in it, but I cant tell.

 

[jsoong]

1. Most fish do fine in stable pH, some prefer harder water. 7.8 is just fine.
2. pH drifts down because of waste accumulation .... NH3 (ammonia) turns into NO3 (nitrates) by giving off H+ (hydrogen ions = acid). The pH drift is a bad thing. You combat that with regular water changes and with adequate buffering in the water.
3. Your pH rise is likely carbonates from something in your tank dissolving into the water. <~7.8 is the equilibrium pH of HCO3.> It doesn't have to be shells, some rocks will do it too. <eg limestone>. This is not necessarily a bad thing. People would deliberately add crushed coral or shells into the tank to increase the buffering capacity of really soft water to prevent the pH drop. So if your pH is stable in your tank, I'd leave it alone. <When it comes to setting up a new tank, if you use the same substrate, you would be wise to let the water sit for a few days until pH stabalized, then slowly acclimatize any new fish to the new pH using the drip method ... ideally over several hours. Unless you are breeding fish, or doing discus or something like that, you can leave the pH at 7.8.>

 

[Egress]

When you add tap water to your aquarium do you see tiny bubbles showing up in the tank afterward? If your tap water is saturated with CO2, you may see a rise in pH as the CO2 is released from the water. You could try testing some tap water that has been agitated well, see if it has a higher pH (like your aged tank water) than your water straight from the tap.