Why is my pH so low? - Questions for a 36 gal planted tank.

For a while now, I've been having some problems in my freshwater planted tank.

We do weekly water changes, and at the beginning of the week, pH will be normal (6.6-7), and yet it never fails to drop by the end of the week (6.0 or slightly lower). At first, we thought we might not be cleaning our filter (had a Fluval FX6 at the time, WAY oversized for the 36 gal we have) often enough to remove the detritus and debris that might be stuck decomposing and lowering the pH. Of course, being lazy (and also having alot of other problems with the filter), we just decided to replace the FX with a hang-on filter that would be much easier to clean.

Fast forward to now, though, and we're still having the same exact issue. pH is high in the beginning and never fails to get progressively lower. From my own research, the cause of this should be high nitrates and decomposing waste that cause the water to become acidic, but there's one problem with this: our nitrates are low and always have been. With the FX, we were always ~30-40 ppm by the end of the week, and now with the new hang-on we're even less (just checked it now, before doing the weekly maintenance, and it's at ~20 ppm) probably because of the increased cleaning frequency of the actual filter.

pH has lowered consistently at pretty much the exact same rate and severity with both filters. Water clarity also doesn't seem to be having an effect, since the lowering happens the exact same regardless of the amount of gunk visibly in the tank or cloudiness of the water. It also isn't my tap water, since it's at ~6.8 pH normally.

Tank info: As of writing this, nitrate is at ~20 ppm, nitrite and ammonia are at 0 ppm and always have been, and hardness is ~110 ppm (our tap water is naturally hard). The tank is overstocked, but the issue also occurred a while back when it was much less full and actually was worse, ending up killing some new fish we tried to add (better maintenance has improved the situation). The tank is well aerated and we currently have a Top Fin Pro Series filter that's rate for 300 gph, though we're planning to add an extra sponge filter for peace of mind.

I've been kind of lax getting to this, since the fish I have in the tank are well used to lower pH by now and I don't plan to add any more, but I'd really like to know why the heck this is happening.

Any help is appreciated!

 

Have you ever tested your tap water than let it sit for 24hrs and retest? Some tap water drop like yours. Next do you vacuum the substrate regularly?

 

hardness is ~110 ppm (our tap water is naturally hard)

1. which one? GH or KH? 2. This is not very high, tbh. Only about 6°.

The predefining factor for the pH is KH. High KH means high pH. If the above means GH, it's to be expected KH is below 5.

You mention Nitrates going up to 40 mg/l, which is not much in terms of acidity (Nitrates are actually dissolved as nitric acid) being able to counter the KH, thus lowering pH.

Anyhow: Without numbers for KH, there is no way of telling what exactly makes the pH go down.

What type of substrate do you use?

Some other comments:

often enough to remove the detritus and debris that might be stuck decomposing and lowering the pH.

Mulm stabilizes the pH in a zero KH-environment and doesn't lower it at all in environments that have measurable KH. So this is based on a misinformation.
Also the pre-mulm phase of rotting botanicals indeed is able to lower pH, although this needs a very low KH (under 2°) to begin with.

One thing it might have to do with: Growing bacteria colonies use up a lot of O2 and carbonates. It is possible you OVERcleaned your tank in certain spots like filtermedia and substrate. The removal of bacteria and archaea colonies is followed by a growth phase that uses up KH.

 

Need to know the KH of the tank and of the tapwater.
Besides nitric acid from nitrates, there's also carbonic acid from co2.

You mentioned filters, but do you water change also? How much water change are you doing?

 

Have you ever tested your tap water than let it sit for 24hrs and retest? Some tap water drop like yours. Next do you vacuum the substrate regularly?

I have tested the water, primarily because I believed it may be the cause of the low pH, but I haven't let it sit first. Let you know what happens with this!

Every water change I do, I make sure to vacuum at least some of the gravel and get any gunk out of it.

1. which one? GH or KH? 2. This is not very high, tbh. Only about 6°.

The predefining factor for the pH is KH. High KH means high pH. If the above means GH, it's to be expected KH is below 5.

You mention Nitrates going up to 40 mg/l, which is not much in terms of acidity (Nitrates are actually dissolved as nitric acid) being able to counter the KH, thus lowering pH.

Anyhow: Without numbers for KH, there is no way of telling what exactly makes the pH go down.

What type of substrate do you use?

Some other comments:

Mulm stabilizes the pH in a zero KH-environment and doesn't lower it at all in environments that have measurable KH. So this is based on a misinformation.
Also the pre-mulm phase of rotting botanicals indeed is able to lower pH, although this needs a very low KH (under 2°) to begin with.

One thing it might have to do with: Growing bacteria colonies use up a lot of O2 and carbonates. It is possible you OVERcleaned your tank in certain spots like filtermedia and substrate. The removal of bacteria and archaea colonies is followed by a growth phase that uses up KH.

The 110 ppm is for GH. Would have listed KH before but I got a little confused with my test strips and forgot the mention it. It was very low, though (not entirely zero, but around 5 like you said) and always has been.

I use standard gravel for substrate, and it's been in use for years in that tank. I do try not to kill off any more bacteria than necessary when I do water changes, washing the substrate in tank water instead of tap to try and keep the bacteria alive. Additionally, the new filter is filled with bio-media that has alot of bacteria growth due to use in my FX-6.

Need to know the KH of the tank and of the tapwater.
Besides nitric acid from nitrates, there's also carbonic acid from co2.

You mentioned filters, but do you water change also? How much water change are you doing?

We do weekly water changes of ~40-50% to get rid of any detritus left in the tank (like I said, it's overstocked).