I noticed a wrong and bad fishy smell to a tank when it was crashing and fish were dying, you could definitely tell something was wrong with the water just by the smell being unpleasant when the water was disturbed and I was removing it to do a large water change. The filter had been turned off for a couple of days without my knowing, in a heavily stocked tank. Plants had held it for a day or two, but big ammonia spike caused a tank crash and lost a lot of fish
One of my tanks is a bit different, my breeding pygmy cory and otocinclus tank. It's my oldest one, and something about it just seems to really work for them. Has half gravel, half sand substrate, loads of live plants including floating, and I let algae grow, and use botanicals a lot to encourage micro critters like seed shrimp. I can see tiny things swimming in the water that I don't see in my other tanks that are less botanical and mulmy. I'm pretty convinced that this is the reason that the pygmy fry survive and grow well in a colony set up, raising themselves in the tank and feeding on those micro-critters and algae in between the food I add, and the otos appreciate it too.
If the plants get a bit wild and overgrown, especially if the floating plants are taking over the surface and there isn't enough surface disturbance, it can smell stronger than my other tanks. The smell isn't bad though, but I'd also describe it as a sort of natural, earthy, unpolluted lake sort of smell. Really hard too describe, but it doesn't smell wrong or bad. Just perhaps overdue for the plants trimming/dead leaves being pulled, and a gravel clean when I do the water change. Thinning out the floating plants and increasing the surface disturbance.
What's the surface disturbance like on your tanks? Maybe it needs some more disturbance to allow gas exchange? But the warm temp and the wood makes sense! Plus, you speed up the fish's metabolism when they're kept warmer, so more feeding = faster bodily functions = more waste products, perhaps? Glad you noticed a difference when you lowered the temp! This is an interesting thread, and useful to know the result when you lowered the temp.
