pH spiked overnight

[Katrina B]

36 gallon freshwater with plants and wood, filter, air stone, heater.

My tank is still new and cycling. The first week I planted the tank and had the filter running. (I didn't know at the time that I could have cycled the tank using bottled ammonia) A week later I added 6 Neon Tetras and started testing the waters using API Freshwater Master Test Kit. A couple days later those fish died, so I added 6 more Tetras. When those also died a couple of days later I went to the internet and discovered Tetras are not the hardiest for cycling a tank. I got 6 Zebra Danios. Even though the internet said they were hardy, they also died. I replaced them with 6 White Cloud Mountain Minnows, who are still alive and I would really love to keep them that way. All of them were/are fed twice a day, morning and evening.

I have been testing my pH, Ammonia, Nitrite, and Nitrates every day since I first added fish. My pH has been in the range of 7.4-7.8 for 15 days. Yesterday it was reading 7.4-7.6. Today when I tested it had jumped to 8.0. Why is it suddenly so high?

The last partial water change I did was on Sunday. I added AquaSafe Plus from Tetra to my tap water. I have been adding Stability by Seachem every day since I got the Danios. I added the Minnows on Monday, which is the last change I have made in my tank. Today is Thursday. The Minnows are still eating well and active. I was planning on doing a partial water change tomorrow (Friday).

For those interested (I don't know if it's important) these are the results of the last 7 days of water tests:
Friday: pH 7.4-7.6, Ammonia .25, Nitrite 0, Nitrate 0
Saturday: Ammonia .25, Nitrite .25
Sunday: pH 7.8, Ammonia .25, Nitrite .25, Nitrate 0
Monday: pH 7.4-7.6, Ammonia .25, Nitrite .25, Nitrate 0
Tuesday: pH 7.4-7.6, Ammonia 0, Nitrite .25, Nitrate 0
Wednesday: pH 7.4-7.6, Ammonia 0, Nitrite .25, Nitrate 0
Thursday: pH 8.0, Ammonia 0, Nitrite .25, Nitrate 0

[Aiken Drum]

Water parameters will be all over the place until your cycle establishes and things settle down.

A few points.

Do you know what your water parameters where at the time of the fish deaths?

Do you know the parameters of your tap water? Let some water sit out for a day in a heated room. This will give the water time to degas and give you a reading of what your tapwaters parameters are. pH in particular can vary over time as CO2 in particular can affect pH and the CO2 present from the tap can be very different to CO2 present after water has had chance to offgas.

Its very unlikely your fish deaths are down to your tank not being cycled. Ammonia just doesnt kill fish that quickly unless you are seeing very high ammonia. ie whole numbers. The most likely causes of deaths in the time period you have been seeing are not using a water conditioner (ill take you at your word you used aquasafe), not acclimating fish properly to the difference between the water in the bag your fish arrived home in and your tanks water, or the fish where sick from the store and the stress of moving them etc was just too much for them.

Dont just do water changes for the sake of doing water changes. You need some waste in the tank to feed the bacteria responsible for your cycle and get it established. Your target should be to keep ammonia + nitrite combined no higher than 0.5ppm. The levels you are seeing dont require a water change and your cycle will never establish if you dont allow them to elevate to some degree.

During your cycle you shouldn't feed them so much. Daily as much as they eat in 2 minutes, or as much as they eat in 3 minutes every 2 days. Even when everything is cycled only feed your fish daily as much as they eat in 3 minutes, or half that twice daily. Overfeeding is not good for fish. A slightly hungry fish is a healthy fish.