Did a tank overhaul kill these fish?

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Dr Peter Burgess contemplates the reasons why a reader's fish may have died following a tank refurbishment.

Q: After a refurbishment of my tank due to overgrown plants and a general tidy up, five Platies and Swordtails died. The only thing added to my tank was 15 small shrimp obtained from a fellow fishkeeper. No other fish have died since but I am still curious as to what caused these deaths. Fish in the tank include Clown loaches, a Royal panaque, a bristlenose, Kuhli loaches, a Red-tailed shark, Dwarf gouramis, a Molly, a Platy and eight Swordtails. Water temperature is 78°F, ammonia and nitrite 0, and 7.9pH.  

All the fish were kept in the tank during the tidy-up which included thinning overgrown plants and cleaning the substrate with an aquarium vacuum, and a partial water change using dechorinator. The first fish died the next day, and the others within a couple of days. The last one to die had its insides missing, leaving an open cavity front to back. Any ideas what might have happened, please?

MERVYN HICKS, VIA EMAIL

A: Peter says: If major changes to the aquarium are planned, such as removing and cleaning large areas of gravel or rockwork then it’s generally best to temporarily relocate the fish whilst this work is carried out. There are a number of things that can potentially go wrong when performing a major overhaul on an aquarium with the fish in situ. Fish, particularly bottom-dwellers such as catfish and loaches, could be accidentally crushed or injured during the repositioning of rockwork and other decor. Deep layers of gravel, and areas of gravel under rocks, may over time turn black due to anaerobic conditions (lack of oxygen with the gravel). When disturbed these pockets of gravel release poisonous substances into the water, risking harm to the fish. 

Replacement tapwater should be dechlorinated and at the same temperature as that of the aquarium water, otherwise the fish could be stressed or harmed by temperature shock. Similarly, adjust the pH where necessary.

In addition, some delicate species of fish may be stressed by a major upheaval to their aquarium, as may sick or very old fish.

But I can’t fathom why several of your Platies and Swordtails died, yet all the other species, including some that I would judge to be more delicate, such as the Clown loaches, Royal Panaque and Dwarf gourami, were unaffected. 

Were the affected livebearers sick in some way, very old, or heavily pregnant females, I wonder, hence making them more vulnerable to the stress of a tank overhaul? 

Regarding the dead fish with its insides missing, I suspect that after it died its carcass was scavenged by other fish, snails, and possibly the shrimps, which consumed the soft tissues and organs. This can happen quite quickly: I’ve observed snails congregating on a dead fish and consuming its tissues within a few hours. Its nature’s way of cleaning up.

I’m sorry that I don’t have a definitive answer — we may never know the reason for these fish deaths, sadly.