There have been a couple of very serious issues raised in the last few posts since my previous. I will start with the most serious, that of relying on plants to deal with high ammonia. Ammonia at any level above zero is harmful to all fish.
This is now scientific fact, so there is no argument. Even a level as low as 0.25 ppm has been proven to cause gill damage in fish. And this damage is irreversible. At best any level of ammonia will stress the fish and weaken them internally, and down the road they usually succumb to some health issue that would not otherwise have occurred. And as far as we have evidence from careful observation, the fish die prematurely.
Ammonia will cause one or more of these symptoms:
- reddish gills, with the gill cover usually held further out from the body;
- clamped fins, later torn and jagged fins;
- red streaks (blood) on the body, which is tissue damage;
- fish gasping for air at the surface;
- lethargy, difficultly swimming, wobbling, or often just lying on the bottom respirating faster.
During any of the above, internal damage is occurring to the brain, organs, and central nervous system. The fish begins to hemorrhage internally and externally, and eventually dies.
As you can see, some of these symptoms occur from other issues too, which is why exact diagnosis of fish disease can be tricky. Preventing disease is therefore much wiser than waiting for it to occur and then attempting to treat whatever it is. And preventing ammonia is key in this.
As I commented previously either in this or another thread, live plants do take up a considerable amount of ammonia. They use this as their prime source of nitrogen, but they also take it up as a toxin. Obviously there is some limit to how much they can take up, and I don't know what this limit is. When I discussed this issue with Tom Barr, a trained botanist whose name some of you will know from his extensive work in the area of planted tanks, his advice was that in most cases the plants, if heavily planted and fast growing, would be able to deal with most usual rises in ammonia such as in tap water at a water change or from a dead fish. But this is minimal; when we are testing levels at 1 ppm or higher we are far beyond this.
So, an immediate water change of no less than half the tank should always be performed if ammonia is determined to be above zero, and this continued daily until ammonia is zero. But the damage is already being done to the fish.
The pH is also significant. In acidic water, with a pH below 7, ammonia is changed into ammonium which is basically harmless. Plants and bacteria take this up. But in basic water (pH above 7) ammonia remains ammonia and is toxic to all life forms, be it fish, plant or bacteria, as levels increase. Most ammonia detoxifiers on the market work by changing ammonia to ammonium (in basic water) but they may have a limited effectiveness, say 24 hours or whatever. Water conditioners like Prime work this way.
Moving to pop's question on corys and other fish showing signs of ammonia poisoning. Corys have a low tolerance to ammonia before they begin showing the above signs, and usually if they do show signs they will be dead within a few weeks if not days. This is why new shipments of corys in the fish store frequently have high losses. I remember a local dealer telling me once of losing all the corys in shipment after shipment until he convinced the supplier to ship them in larger bags with only a couple fish in each. The ammonia poisoned them fast. Molly have the same intolerance. But "intolerance" is rather pointless, since any level of ammonia is harming the fish somehow, and they will not recover.