Sudden fish loss.

[cobden28]

I have a 4 ft planted tropical 'community' tank in my living room, containing honey & pearl gourami (2 of each), platies, guppies, a pair of bristlenose catfish, six Styerba's corydoras, 3 dwarf chain loaches, and half a dozen of each of neon & black neon tetra's. Up until last week I also had half a dozen glowlight tertra's until I lost four glowlights on the same day - and I can't quite work out why!

I regularly test my tank water weekly, using the Tetra 6-in-1 test strips and the Tetra ammonia test kit. Up until a month ago my weekly test results had been as follows, consistently, for a couple of months:-

Nitrate 0
Nitrite 0
GH 16
KH 20
pH 7.6
chlorine 0
ammonia 0

A month ago, in mid-May, the pH shot up to 8.0 and last week and the week before the nitrate level also shot up, from 0 to 10. Three days after the nitrate level rose to 10, I lost a platy and a glowlight tetra with no aparrent symptoms showing previously - they were just DEAD when I came downstairs in the morning. I did a partial water change of 4 x 12-litre buckets. Two days later, on Tuesday of this week, I came downstairs to find 4 glowlight tetra's dead leaving just two glowlights left in the tank.

When I tested the tank water this afternoon the nitrate level was still at 10 and the pH had risen to 8.4, the highest its ever been. I know we live in a hard-water area but 7.6 is the normal pH for my tank and I haven't the faintest idea why it should suddenly have increased. I did a 50% water change in my tank (10 x 12-litre buckets) and a couple of hours after the water change the nitrate level had reduced to a much healthier level of NIL and the pH had reduced to 8.0.

I change the filter pads regularly in accordance with the manufacturer's schedule (Juwel), always use a water conditioner when I top up the tank water levels, and all the other fishes in the tank appear in the pink of health. I really am puzzled as to why I lost four glowlights on the one day - can any forum members throw any light as to why this may have happened? Did I do the right thing in doing the 50% water change today? What can I do to make sure this sort of thing doesn't happen again?

[Carylnz]

I am intrigued that a well established aquarium has 0 nitrates. How accurate are your strips? Liquid tests are usually more accurate. Of course, if you have some big heavy root feeding plants, like some of the swords, they will suck up all the nitrate.
Have you checked the tap water? Perhaps the council did something recently to alter the pH and/or hardness. Ours flushed chlorine through the other day after they cleaned out the reservoir behind our house. Since we do not usually have chlorine in our water it was a big shock. Luckily I had not done any water changes.
Nitrates of 10 are nothing. Mine are usually around 40ppm as that is what the tap water has!
What is the pH of the tap water. Both immediately and after standing 24 hours?

[cobden28]

I use the standard Tetra 6-in-1 test strips and whilst I can only assume they're reasonably accurate, what is of concern to me is why the nitrate level suddenly shot up to 10 after having consistently been at 0 for weeks on end .

The pH of our local tap water is, I know, hard at a normal reading of 7.6 and again what puzzles me is why the level suddenly went up. I'll test a sample of our local tapwater first thing tomorrow morning, and again when the sample has been standing for 24 hours as you suggest. Although when I do top up the water level in the tank, it's straight from the tap but with an appropriate dose of tapwater conditioner added. Should I be leaving tapwater to stand somewhere for 24 hours before topping up the tank - that would be a problem as i currently don't have either any spare buckets to use for this nor the space to leave them while the water 'conditions' itself (that's why I use the tapwater conditioner chemical in a bottle)

[Carylnz]

Having water that requires no additives of any kind, I can't help you there! As far as I know it is fine to add them as you do the water change with no need to leave it standing.
By seeing what the test results are straight from the tap, and after standing, I thought might give an idea of whether there is a problem within the tank or the tap water itself.

[cobden28]

I'm just about to go test my local tapwater now, but before I do I should point out that the tank water is always crystal-clear, the plants are thriving and apart from these sudden unexplained deaths recently I'd say my fish are usually happy and healthy.

In fact on the very day I discovered the four dead glowlights I also discovered four newborn guppies and three black mollies born in front of my eyes! Normally none of my livebearer fry survive as the tetras keep the population under control.

[Carylnz]

Water clarity does not necessarily equate to good quality
You do lose the occasional fish without knowing the reason, like the platy, but is it possible the tetras were old? If you buy a group of fish it is likely they are siblings and all the same age. This means they tend to die around the same time as well. Unlikely in this case but another thought.
Were the deaths after a water change? It is possible there was something in the water at the time and the tetras were most sensitive to it.