An odd problem with fish and plants

[Poecilid]

I have a 1320 gallon water system serving two 530 gal tanks. For the first 15 months the system thrived. Plants grew like weeds, fish grew and bred, and all was right with the world. Quite suddenly, the plants stopped growing. They didn't turn yellow but just sat sulking. The fast growing ones like Ambulia became spindly, and the Amazon Swords lay flat and sent up no replacement leaves as old ones died. The fish stopped breeding. They still eat well, and look healthy, but are less active. pH is 6.8, water slightly soft, no nitrate, nitrate, or ammonia problems. I had a friend with Black Mollies that she was keeping successfully in soft, acid water. So I'd added some of hers to my tank. When the problem started a few of them lost their colour (becoming grey). The tanks are lightly stocked with kissing gouramis, moonlight gouramis, golden gouramis, red-tail black sharks, angels, golden barbs, swordtails, kribensis, and albino coydoras in one, and platies and mollies in the other. I assumed the plants had used all the nutrient when the problem started, and added plenty of plant food. It had no effect. I do my water changes with rainwater. I can only guess there is some toxin in the water but have no idea what to look for.

[Delapool]

Was thinking the same of checking water first. Has there been any change in water chemistry? That might be a proxy for some other element.

Um, next one would be any change in equipment used. And if you have treated tank with anything. Some meds or liquid carbon I’ve found plants don’t like.

[Poecilid]

No, I haven't added anything new that could change the water chemistry. Maybe there was something in the sand I used, or maybe something got into my rainwater tank (which is sealed). The only thing which regularly gets added to the tanks is my body as I snorkel around cleaning the front glass. But, again, I don't use any toxic substances around my body... so, still stumped.

[Poecilid]

I've kept fish successfully for fifty years without using meds or water conditioning chemicals. I've used algaecides occasionally... but not in this system.

[SwampeastMike]

Have you emptied, flushed and cleaned (however possible) the rainwater tank?

Since you get in the tank regularly any change in soap, shampoo or unless you're nude, laundry items?

[Magaratz]

I’ve realized with all the Covid safeguards we’ve been through that I have come close to doing maintenance after using an alcohol product on my hands when I have been out and have almost forgotten to wash hands thoroughly before messing with my tank. Are you possibly cleaning the snorkel in a bleach type solution?

[WildFish99]

Sounds a little like theres not enough co2 for the plants, but Im very unsure why the fish would have been effected as well. Did lighting or seasons change since they were active?

[Poecilid]

Thanks for all the suggestions about the possible causes of my problem. I'm pretty sure it's not Co2 deficiency, as the fish are also affected. I think I'll have to assume it was a chemical contaminant, but I may never know what.

[jake37]

I had a weird problem not totally unlike yours. The problem i had was that my substrate effectively went bad. I had used a fine grain substrate about 3 inches deep and eventually a large amount of sulfur was generated from anawhatever. If you still your substrate slightly do you get gas bubbles that smell like rotting eggs ? If so then you might need to replace your substrate with something more porus.

[WildFish99]

Originally Posted by jake37

I had a weird problem not totally unlike yours. The problem i had was that my substrate effectively went bad. I had used a fine grain substrate about 3 inches deep and eventually a large amount of sulfur was generated from anawhatever. If you still your substrate slightly do you get gas bubbles that smell like rotting eggs ? If so then you might need to replace your substrate with something more porus.

If your having gas form in sand, usually the best thing to do is only vac the very top of the sand and to not disturb it. That or have livestock who love and dig in sand a will keep it all aerobic instead of it turning anaerobic.

[jake37]

I was only vacuuming the very top..... I know the theory is that if it is not disturbed things should work but for whatever reason.... - oh yea - hum - the seals on the tank broke (an aqueon 29) after 1 year so i had to replace it - so yea the substrate was shoveled out - BUT it was having problems before the replacement and I've had this issue with this specific substrate in other tanks so it is reproduceable and i've ceased to use it.

Originally Posted by WildFish99

If your having gas form in sand, usually the best thing to do is only vac the very top of the sand and to not disturb it. That or have livestock who love and dig in sand a will keep it all aerobic instead of it turning anaerobic.

[Poecilid]

I have a 4 inch substrate of light sand. It gets dug up when I'm walking in the tank during cleaning, but I never get any bubbles or odours from it.

[jake37]

If there are no large bubbles or smell of rotten egg (which would be quite strong and obvious) then your issue is not the same as mine.