Who Gives A Fish Forums

I've got a puzzling situation on my hands with my 90-gallon discus tank. It's been running smoothly for about three years with plenty of live plants, substrate, and a stable environment. My maintenance routine has been consistent - 40% water changes every week, dechlorinated with Prime.

The tank's inhabitants include a school of medium-sized discus, a few cardinals and rummy nose, and a trio of rams. We've found that keeping the temperature at 85-86F is key for the rams.

Last November, the pH dropped from 7.2 to the low sixes, accompanied by an ammonia spike. I recall researching that at lower pH, ammonia becomes less toxic as it exists as ammonium. I chose not to buffer the pH, figuring a higher constant pH is better than a fluctuating one.

Until November, the tank was stable with a pH of 7.2, zero ammonia, and nitrates around 10 ppm. Then, the ammonia dropped to zero, but the pH remained low at 6.5. This became the new normal.

Recently, I noticed a cloudiness that worsened after a water change. The water parameters were fine, but the ammonia spiked to 1 ppm. I'm hesitant to do more water changes, as it seems to reset the cycle and make things worse. I've held back food for a day or two, but the fish seem fine.

Something's crashing the biology, and I'm not sure what. I did a thorough gravel vac during the last water change, but I haven't touched the canister filter. I've also unplugged the UV and added beneficial bacteria culture to help the good bacteria outcompete the cloud.

I'm taking a wait-and-see approach to see if the cloud clears up and the ammonia drops. Any ideas or similar experiences would be appreciated.

My water change regimen has been the same for three years: I siphon water out of the tank and replace it with dechlorinated water from a 36-gallon plastic bin. I've started wondering if the plastic bin might be breaking down and causing issues.

Re: Bacterial Bloom

A few observations Andy...

I think I see a potential issue - you mentioned not entering or fussing with the filter (canister), which has been in place the whole time. To me, that means any debris or waste within the filter is never actually cleaned out. Most folks I know clean their canister filter at least once a month. I'd say giving it a good cleaning might be a step in the right direction.

If I were in your shoes, I'd consider doing two water changes a week until you get the cloud under control. Also, if you do end up doing a deep clean of the substrate, just be aware that disturbing a deep substrate can trap gases, which can be harmful to the fish.

Re: Bacterial Bloom

Hi and welcome. I've been thinking, with 40% water changes over three years, it's likely the tank water has been depleted of its KH, using up its bicarbonates and making it harder to stay stable. The remaining 60% of old water each time has probably built up other nutrients like phosphorus. You've got a planted tank, so a low nitrate reading isn't necessarily a good indicator - it's like the old 'tank syndrome' issue. A good rinse of your filter media would help get rid of the sludge that's built up, smothering the oxygen and preventing it from reaching the beneficial bacteria. It's possible the pH crash hindered the beneficial bacteria. More frequent water changes and a deep clean of the substrate could be the way to correct the issue.

Re: Bacterial Bloom

Just another consideration here. I'd suggest spreading out the water change over the course of a week, rather than doing one large change and cleaning. The chemical difference between the new and old water, particularly the buildup of TDS and DOC over time, can be quite significant. You want to avoid stressing the system with sudden changes, which could exacerbate the issue.