Water PH 4.0

Are you sure the reading is correct?? 4.0 is acid, veryyyy acid. Are you sure you have a correct kit??

If there is too much "gunk", too much fish, ect, you will need to up the water changes or the size of the water change,

To answer the question of preferred pH for goldfish, they can tolerate 4.5–10.5, but prefer 5.5–7.0.

I'd also make sure the reading is accurate, maybe take a sample to your LFS to double check... As well as up the water changes. I'd do that 5 gallons out of 36 for two large goldfish at least twice a week...

a 36 gallon tank with goldfish and you do a 5 gal water change every once in awhile??
how often is "once in awhile"?

the reason I ask... goldfish are considered messy fish... which = lots of waste . this in turns leads to waste build up in the tank which often leads to unstable tank parameters(usually acidic water) as the buffering capabilities cannot keep up with the acid forming from nitrogenous waste build up.
I'd suggest you look into this as a possible cause to your water chemistry issues.

I agree. Most goldfish aquarists would say two goldfish is too much for a 36g, depending upon the goldfish perhaps. But regardless, you must do larger water changes (I would say half the tank, no less) and at least once a week.

did you calibrate the meter with a reference solution? If it is digital and not liquid then you need to calibrate before using because light on the probes or fluctuation in temp can cause the meter to read pH incorrectly. A lot of these meters are more trouble than they are worth unless you go with an expensive water proof meter like HM or blue lab. If you dont want to drop the cash then buy a pool test kit or API fresh water master kit (although with API i think you can only tell if the sample is 6.0 or higher).

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