I've had my Oscars 6 months now. No problems whatsoever, water parameters have remained constant and within the acceptable range.

However, I positively dread performing water changes.

Initially my baby Oscars were housed with 2 clown loaches and 2 young pictus. All was well until I performed the first water change. I changed 40% that first time and used Stress Coat to condition the water. I've tested our tap water and aside from being fairly hard it doesn't register anything untoward after being conditioned. All was well for the rest of that day but the following morning one of the pictus was dead, the other almost so. The clown loaches looked ill and the Oscars listless and gasping. Fortunately I have room to move them so everyone was relocated to alternative tanks and all perked up perceptibly within a couple of hours. I should add that I have two other stocked tanks and performed water changes in the same way that day - with no ill effect in those ones.

I tested the water in the now empty Oscar tank... everything was fine, including the test for Chlorine and Chloramines which was negative. I left the tank running for a couple of days, then introduced a few Tetras from my community tank. When they were still fine after a few days I felt it safe to return the Oscars and the remaining tank mates to their original tank. All was well.... until the next water change. This time I performed a smaller 20% change. Tested everything. Everything fine. Everyone fine... until the next morning. 2 dead clown loaches and a dead pictus this time. And miserable Oscars. I moved the Oscars back to the safe tank again, and left their main aquarium running for a couple of days before reintroducing them. All was fine again.

We're now 5 months on and still having the same problem after every water change. No one has died thankfully, but the fish look listless and gasping the following day and for a few days after. I haven't a clue what the problem is. The other two aquariums in the home are done in the same way, on the same days, with no ill effect to them.

The water temperature is 25 and I use a Newave 3000 circulation pump to keep the water circulating and aerated.

So what can be going on? I need to keep the Oscar tank clean but dread the consquences every time.

Instead of Stress Coat maybe try a dechlorinator such as Prime or something similar! Maybe the stress coat is bad???

Nicki--Sue wrote:


Been using it for years! But yes, thought that too and have used various alternatives over the past months and tested the water after using them. They're all working, but the different products aren't making a difference to end result. Sickly fish.

In desperation I've started to perform a water change in my large community tank a couple of days before the Oscar tank is due, and syphoning off water from that tank to replace the Oscar tank water I've taken out. Seems to work better than using fresh tap water, but this last time my smaller Oscar looked a bit 'off' afterwards although the other fish were fine. I'm starting to worry that this method isn't going to 100% reliable either or a suitable long term cure.

It's driving me nuts TBH. And making me feel very anxious. Something I've not experience in my 30+ years of fish keeping.

i would guess you worked out the amount of stress coat wrong. prime is a far better option, as it removes more "nastys" than any other water treatment, and covers a larger amount of water for the money.

2nd guess is temp of new water
3rd guess is a ph/hardness change. check alkalinity GH?KH?

are you putting the new water into the tank and treating. you could try turning the filter off and hand mixing the conditioner before turning the pump back on. as long as its not off for a long time (over 30 mins) you should be ok. some areas have more chlorine in the water than others, you could also try adding a little extra conditioner just in case.

i don't add my water to the tank. I treat the new water before I pump it into my tank and i always consider this a better option. this way safe water only ever ends up in the tank.

Many thanks for the suggestions



No, I definitely didn't make a calculation mistake. It's too easy to use for that. Plus, none of the other products have helped and my other aquarium communities aren't affected by the same water, treated in the same way. I have a measured 15 litre bucket which I exclusively use to fill with tap water. I add the dose of conditioner midway through filling each bucketful, stir it thoroughly when filled and let sit for a couple more minutes before adding it to the aquarium. I switch off all filters, etc whilst performing the change.

Initially we wondered if there had been a spike in the additives in our tap water over the weekend, as someone suggested the water companies add extra prior to the heavier use over weekend periods. I haven't a clue whether this is believable but since I inevitably perform my water changes over the weekend, I did consider this and wonder if the 'normal' dose of water conditioner might not be enough. However, that doesn't account for why my other tanks aren't equally effected.



Yes, I wondered about this but wouldn't the negative effect on the fish be quicker? They're fine initially, until the effect seems to hit them hard between 8 and 12 hours after the water change. And the Pictus and Clown loaches actually died. They were previously living in one of my other aquariums - the loaches for 3 years, so something quite dramatic must have affected them. I was mortified.

Could a minor temperature change cause this?



Yes, another possibility I've considered but having tested the established aquarium water with the conditioned tap water I am adding, there's no difference in the results. I also keep Malawi cichlids so test, and treat their water with various products when perfoming water changes. However, I don't add salts or use any water softeners or buffing agents in my mixed tropical or the Oscar tank.

Should I?

Only thing I add, now I think of it, is Stress Zyme; A biological filter additive containing live bacteria.

Have just bought 500ml of Prime! It's about the only product I haven't tried so far. LOL. Sounds like useful stuff, so thanks for the recommendation.

..... just wish I didn't have a brand new 5 ltr of Stress Coat sat here! Typical

just some thoughts...
1. Use the prime (let sit before turning back on filters)
2. Do less percentage change at a time... 15-20%
3. Make sure temp of water is almost identical to that of the tank.***
(you can fill up a large bucket and check the temp; if needed add more hot or cold; you can also add the prime at this time also, before it goes into the tank.

It is unusual though from your comments. Good luck with the new water changes, fill us in on the next one.

i am also baffled by your post. i will try to simulate your water change
procedure and will share the findings later. definitely, there is something
wrong that triggers the death of the helpless fishes.

Many thanks for the suggestions.

I'll wait for the Prime to arrive (bought it online) and will buy an accurate thermometer for the refill bucket in the meantime. It's the only variable I haven't specifically checked before adding the water. I generally guestimate using my clean hand, and check the digital reader in the tank as I'm proceeding with the refill. Because my bucket is only 12 - 15 ltrs it takes a few bucketfuls to replace the water level.

I think I'll also test my tap water prior to commencing each water change. Have done this randomly in the past, but I cannot help thinking it MUST be something IN the water affecting the fish for the first few days. Whatever it is, it dissipates on its own within 3 or 4 days as the fish perk up by then. Just cannot fathom what it is as it's nothing showing up with the API Master Test kit, nor the chlorine/chloramine tests.

Anyone think standing some conditioned tap water for a few days worthwhile? I haven't tried that yet. At a college I used to lecture at, the animal care manager would simply stand unconditioned tap water in dustbins and use that for the numerous tanks the college kept in the animal care unit. The water supply there only used chlorine and it seemed to work for them. My water supply is chlorine only too, so I wondered if standing the conditioned water for a few days might help in this instance?

i'm trying to figure out why but honestly.... i can't see any reason
why? what comes in my mind is you may have shut off the filtering system
for too long where good bacteria may have died and cause ammonia spike.
loach for one is very sensitive for sudden changes but you had keep them
for three years, thus you are more experience than me.

the only probable cause might be the sudden temperature change that could
have cause cardiac arrest; or too much nitrogen due to high pressure circulation
that can suffocate the fish. however, too much nitrogen shows scale falling
gradually first, not instant dead fish.