pH Drops After Midnight/Early Morning

[fddlss]

Hi, I recently bought a Pinpoint pH Monitor that has been calibrated and I compared the results with the API liquid test kit, to be sure. I have African cichlids and I would like to keep my water's pH, or I should say hardness higher, but most importantly, stable.

KH is about 5.5 - 6 dKH
GH is 8 dKH

I've noticed that my pH moves up and down depending on the time of the day. This is what I've noticed:

Little after Midnight (around 2 AM), early morning (until 11 AM or so) 7.61

Late morning, afternoon 7.75

Late afternoon, night 7.81

Is this normal?

FYI, I have glass lids which stay closed all the time, I run an airstone in the sump, Kroalia air pump 1400, the tank is 120 gallons and it currently holds 9 Malawi cichlids all of them being under 5".

The tank has been set up for around 4 months and with fish for about 2 months. Fishless cycled 3 months ago.

I perform weekly water changes of about 20%. My nitrates stay under 20 PPM.

Also, my old light fixture broke, so I run the tank for about two months with no light, just natural light from the living room windows but no direct sunlight, and I had noticed a few spots of brown algae in the sand in the past, but I'm talking about less than half an inch of brown algae in the whole tank; However, now that I got a new light about a month ago, brown algae has been growing a lot, everywhere, during the last two weeks. This is my light schedule:

3 PM - 9:45 PM - Daylights 2x 10,000K T5 HO 54W
9:15 PM - Midnight - Actnics 2x T5 HO 54W
24/7 (except for when Actnics are on) 4 blue lunar LEDs

The fixture is a 48" Coralife Aqualight T5 HO.

I know lighting has nothing to do with pH or hardness, but I'm still giving you these information just in case. It doesn't hurt.

Maybe CO2 and the lids being closed has something to do with the swings in pH?

Thanks.

[dskidmore]

Plants/algae eat CO2 when the lights are on, and raise the pH. When the lights go off, they stop eating up the CO2 and pH falls. If you have more areation, the water more closely track the room CO2 levels, which unless you have a lot of plants in your house should be more stable.

[Shadowraven]

Agreed. The change you are seeing has to do with the amount of CO2 in the water. This is perfectly normal, especially with your plants. Your pH looks good, so I'd say that there is nothing to worry about. However, if you really want to buffer the water to keep the pH up you can use some substrates, shells or rocks sold for that purpose. I don't see the need for that in your situation though.

[fddlss]

Thanks guys! Actually, I don't have any plants yet, just brown algae that started to grow about two weeks ago. Could the situation be the same even with no plants? Maybe because of the lids being closed? Also, I'm already using crushed coral and a few sea shells to buffer the water and pH went from 7.5 to 7.8 and KH from around 4 to 5 dKH.

[dskidmore]

Originally Posted by fddlss

Thanks guys! Actually, I don't have any plants yet, just brown algae that started to grow about two weeks ago.

Originally Posted by dskidmore

Plants/algae eat CO2 when the lights are on

Algae will do it too. Anything with chlorophyll will, even corals to a lesser extent.

[fddlss]

Ok, so I guess this is perfectly normal! Basically, the light is making the algae grow and when they are on the algae absorbs CO2 therefore pH raises. This is brown algae, and I've heard that more light should reduce brown algae and bring in green algae; However, I've seen an important brown algae increment since I got the new lights. I also read that brown algae could be a water problem (phosphate, silica, nitrate) but I perform a 20% PWC every week so I don't think nitrate is the case. Maybe a little overfeeding, but I don't think so either.

I know that pH swings are not good for fish but this is not that big of a swing, is it? from 7.6 to 7.7 and then to 7.8 in a 24 hour period?

[Shadowraven]

No it is not. It is certainly not anything to worry yourself over. If you were talking about a whole point change, then maybe worry, but not this.

[fddlss]

Originally Posted by Shadowraven

No it is not. It is certainly not anything to worry yourself over. If you were talking about a whole point change, then maybe worry, but not this.

Cool. Thank you.