Do I even need to do a water change?

[David R]

Thought I'd monitor the pH over the course of the week to see what it does. Left is the tap water, right is the tank (about 6.4 currently). I think if I'm going to do larger water changes I'll need to start pre-buffering my water too. If I did a 50% water change I'd halve my total hardness, which I don't think the fish would appreciate!

 

[duanes]

Simple tests like pH are easy to fluff off, but their results are very significant.
I suspected that with your low TDS, that the buffering capacity of your water might be low.
Have you checked alkalinity? This in tandem pH would give the best indicator for its ability to buffer acids.
My normal tap water alkalinity is 90-100ppm, but a week in the tank can drop it significantly between water changes, even with a conductivity of 500-1000mS.

The pre-buffering idea for make up water seems sound.
Because of the pothos use, nitrates are obviously not a problem, but pH swings could be, as uric acid from normal metabolism and fish urine build up over a month.
This could be reason enough to keep your more frequent water change schedule, as opposed to a large monthly change.
Again a bit off topic/sort of
my thought in the growth experiment, is that the dominant fish in any group of 1 species, may produce a growth inhibiting hormone, in order to keep it dominant. Because this hormone may be species specific, it may not effect other species,
and in the confines of an aquarium, with low water turnover (lack of water changes), can be significant.

 

[David R]

I suspected that with your low TDS, that the buffering capacity of your water might be low.
Have you checked alkalinity?

Correct, the mineral content of my tap water is non-existant! Like most of rural New Zealand, we collect rainwater off our roof which is stored in a large tank to supply the house (and aquarium). I haven't bothered checking the alkalinity of the tap water as I know there won't be any, though it would be different if we had a concrete tank instead of plastic. I have a basket of coral rubble in the sump and several kg of beach sand [shell] in the tank to help keep the pH stable and provide a bit of buffering capacity against the large amount of driftwood in the tank. Though more recently I've been wondering what other minerals my water may be lacking and thinking more about working out a dosing regime of trace elements.

I'll check the alkalinity of the aquarium once I've finished my morning coffee.

 

[David R]

Hmm, guess that coffee hadn't kicked in yet, I was thinking of hardness but said alkalinity...

Anyway, KH and GH are both very low, KH ~10mg/L, GH ~20mg/L. I'll chuck a bit more coral in the sump for a quick fix, but will start investigating buffering recipes to dose the tap water with.

 

[duanes]

Seems your water is perfect for the soft water S Americans that I can't possibly keep.
I've always wanted to keep Uaru fernadezyepezi, but with my liquid rock, it just isn't practical, or healthy for that species.

 

[David R]

Yeah it kills me that I have the perfect set up for a group of Uaru F and Satanoperca daemon yet neither species is available (or ever likely to be) here. Is the rain water not clean enough to collect for aquarium use in Milwaukee? I guess it would be frozen for a good chunk of the year!