Why Does Ph Get Lower During Fishless Cycle?

[Gremily]

Hi, I've been fishless cycling my tank for around 3 weeks now (with mature media added) and I decided to to a big water change a 2 days ago because the pH was low (5-5.5) and I read that a low pH causes the bacteria to go dormant. Already the pH is getting low again and I'm worried it's going to make my cycle take longer. I've fishless cycled two tanks before and the first went very slowly towards the end (I'm guessing due to a similar pH problem) and the second also suffered from a low pH and was also slow in later stages.

I'd like to know if anyone knows the cause of this drop in pH. The tank is a Juwel Rekord 120 (built in filter/heater, 120 litres), the only thing in the tank besides the water is gravel and I have the temp around 28C with the filter outlet pointing towards the surface. Tap water is ~pH7.
Does anyone know what could be the problem, I've read that the pH will rise during a fishless cycle so this is quite puzzling!

 

[dthoffsett]

Cycling can cause PH fluctuations either direction. I can't remember the science behind it, but I believe it's caused by the high levels of ammonia.

 

[Musho3210]

the nitrogen cycle produces acids and if you dont have any buffers or dont have much buffers, the ph will lower. Check your KH of your tank water and your tap water.

 

[Gremily]

Ok, thanks, so is adding some crushed coral to the filter a good idea?
I don't have a KH test but I think water round here is supposed to be quite soft.
The other two tanks cycled eventually so I guess this one will too, it's confusing that the pH actually goes down as ammonia is really alkaliney.

 

[Musho3210]

ammonia water is alkaline but the production of ammonia-nitrite produces acids.

NH4=NO3

take out 4 hydrogen molecues and add in 3 oxygen molecules. The 4 hydrogen molecules are acids

I think thats what it is, correct me if im wrong. Im never any good with chemistry