FISH KILL - Mixing Conditioners?

Following a water change, I have a single tank that had a major fish kill. I need a little help understanding. I did my weekly water change in both 55-gal tanks and my 10 gal tank. I have used "Ultimate" water conditioner (1 ml / gal) for years. On the last tank, I ran out of "Ultimate" and had just purchased "Big Al's Multi-Purpose Aquarium Water Conditioner" which I was planning to try (save a buck). So the last 8 gal of make-up water, I used the 'Big Al's". I went about my other chores around the house and noticed at the night time feeding, the fish (African cichlids - yellow labs and blue johannies) were shy and did not frenzy feed like usual. I had to go out of town the next morning and before leaving noticed EXTREME distress. The larger fish (5-6 inchers) where on sides, barely moving and gasping at the surface and multiple smaller fish were dead. All other tanks are fine.

I suspect the new conditioner. I thought the Big Al's was the similar chemical used to remove Chlorine and neutralize Chloramine just like the Ultimate. There is a "Contradiction" listed on the Ultimate which says "do not use with permangante-based or chlorite-based treatments". I thought those were used specifically for things like copper and lead chelating and chlorite was used as a santizier, so I really don't think that Big Al's falls into that category.

I am at a loss. Was able to rescue the larger fish (saved) by moving to the other 55 g. (The pictus catfish are not happy in that tank.) I thought I would come back from out of town to an complete kill in the tank, but those that survived the first 24 hours, have made it.

I changed a total of about 18 gals in the 55 gal. The first 10 gal treated with Ultimate and the last 8 gal treated with Big Al's. Nothing else is changed. I do not suspect bucket contamination since my other tanks are fine. No other water params are outside what my tanks usually run.

BTW - the avator fish survived. He is now about 6 inches. Also, here is some tank data that I am sure you will ask about:

Filtration: Modified AquaClear 110 (HOB) with half sponge and the roots to rhodadrium (?) nitrate sucking plant and SunSun canister (1400 l/h) - No media was changed or cleaned this week.

Params - 0 NH3; 0 NO2; 160 NO3 reduced to 8 NO3 after the change. (That is my normal NO3 levels as this is an overstocked African tank). pH is a highly buffered 8.2 that I gave up trying to change years ago.

I have the chiclid Eco-sand has a substrate with lots of natural rocks. When I run my fingers through the substrate after scraping the glass before the water change, I did not notice any extra bubbles coming up (hydrogen sulfide check I perform every week when I do the water change to avoid issues).

I have had this tank running since 2006 with no fish additions since 2007. In that period of time, I have lost a total of three fish, which I thought was due to fighting. Overall, everyone gets along pretty good, now.

Oops. 80 ppm NO3. ALSO, the plant is a potho (in the AC110).

But...... Folks, I have not run a pre-change set of params in a long time. In fact, I rarely test on an established tank. And this tank and fish population has been EXTREMELY stable for multiple years. It is not uncommon for my NO3 to be as high as 160 BC. I tested after the disaster. I guessed at the pre-number because I thought it may be important that I my cichlid tank usually has a higher NO3 level than most people feel comfortable with. I found that changing 15-20 gal a week had a large affect on the NO3, but as I increased the volume of the change, it had less impact. My high NO3 (that I have been running with this fish population for years) was easier to control with the addition of the potho plant roots sharing the space in the AC110. I cut the sponge in half, quit running charcoal and the plant grows out of control. It helped with the NO3. Maybe if I tested before the WC's now (after potho).

Please, I need help with the fish kill.

I changed water in three tanks. Only the final tank had problems. The one that I used a new chemical. I do not think I have a problem with the source of water.

I'm really concerned that I either have a bad jug of the Big Al's Multi-purpose Aquarium Water Conditioner or it is incompatible with the "Ultimate" conditioner I usually use. The "Ultimate" conditioner has a "contradiction" listed as "do not use with permagante- or chlorite-based treatments". Does anyone know what those are? I thought that permaganate is a chelating agent that binds some toxics into a chemical form that does not react. I thought that chlorite type chemical would be used to kill biological organisms such as bacteria. So I would not think that these would be in the water conditioners. I don't know if I should do more water changes with "Ultimate" or "Big Al's". I do not want to duplicate the fish kill.

Yes. The filters were running. It did seem like an oxygen issue. I am afraid of a hydrogen sulfide release from the sand, so I mix the sand each week and check for gas pockets released. I always get some fine bubbles when I do this. This is why I agitate the substrate before the water change. That way if I get a release, I am ready to start diluting. I have some MTS in the tank that burrow through. This week, I did not get any big bubbles or anything out of the ordinary. I did notice that during the distressed period when the fish were dying, the MTS had come out of the sand and were making their way to towards the top of the tank. I agree that it sounds like a lack of oxygen.

Since I was out of town, when I came back, the remaining fish are acting normally and fed well. The snails are back into the sand and not visible. I did not change the water, because I did not have any conditioner that I trusted. I also think I had some kind of chemical reaction. That is why I wondered if the new chemical may have been permanganate based or chlorite based and maybe the reaction temporarily dropped oxygen? I don't know. I wrote an email to "Big Al's" to find out what the chemical is. I was a little surprised to see that there is no listing on the jug that as an ingrediate list.