There are any number of reason why a low level of ammonia such as .25 ppm may not be real. The first and most obvious two are either an expired test kit or user error. But if we eliminate these as the cause there are still other reasons which are common.
There are things in our water which can cause a false reading, especially a low level one. Iron is one such thing, and there area a few others.
Next, dechlors that also neutralize ammonia can:
I am using Prime® to control ammonia but my test kit says it is not doing anything, in fact it looks like it added ammonia! What is going on?
A: A Nessler based kit will not read ammonia properly if you are using Prime®... it will look "off scale", sort of a muddy brown (incidentally a Nessler kit will not work with any other products similar to Prime®). A salicylate based kit can be used, but with caution. Under the conditions of a salicylate kit the ammonia-Prime® complex will be broken down eventually giving a false reading of ammonia (same as with other products like Prime®), so the key with a salicylate kit is to take the reading right away. However, the best solution ;-) is to use our
MultiTest™ Ammonia kit; it uses a gas exchange sensor system which is not affected by the presence of Prime® or other similar products. It also has the added advantage that it can detect the more dangerous free ammonia and distinguish it from total ammonia (total ammonia is both free ammonia and non-toxic ionized forms of ammonia).
What gives one a good clue that having .25 ppm of ammonia in their tank on a persistent basis would be virtually impossible to create intentionally. if the reading is accurate one has to assume one of two things. One a tank insufficiently cycled should have any measurable ammonia. if one is cycling that .25 can not stay constant, it must either go up or go down. Not being cycled means it should rise as the fish will be making more ammonia than the bacteria can handle and the ammonia rises until the bacteria multiply.
If the tank is at the point of cycling where ammonia is going down, then that .25 should disappear pretty fast- hours and not many of them. So, your .25 should either rise or fall, but it should not stay they the same, unless it is not a real reading.
Next, if you have ammonia in the tank not yet being handled or due to something harming the bacteria, you should be seeing nitrite as well as ammonia. But you read that at 0. Since what can harm the ammonia bacteria should also affect the nitrite ones, if is not common to see ammonia levels not followed by nitrite levels. So the lack of nitrite suggests the ammonia reading is not real.
Finally, the end product of the cycle (in the absence of denitrification) is nitrate. You most certainly have that. So we can also conclude that the cycle is working in your tank unless that nitrate is coming in with your tap water. bear in mind that ammonia can evaporate from water but nitrite and nitrate cannot.
Fially. the best clue of all here is the fact that only some of the fish are having issues. if the cause were ammonia it should be affecting all of the fish to some degree, This is especially true since it is a single species. Normally, different species can tolerate different levels of the three nitrogen compounds with which cycling is involved.
There is one thing about cycling and the related issues which is important to remember. There is established science surrounding this stuff. We know how fast the bacteria can multiply, we know what types of bacteria or typical in our tanks. We know that establishing the cycle follows a series of steps. The only difference is how fast any tank muight cycle. We also know what can stall a cycle or help to accelerate it.
Finally, the scientific method assumes the possibility that what we thought we knew is actually not right because research finds the fault and then works to establish the "new" facts. But at any given time the science is the best we have with which to work. Perhaps the best part of science and research is the fact that for any study to reach a valid conclusion requires that if another group performs the same experiment that it will yield the same results.
What this means for cycling is that it is possible to determine what may be wrong if we have all of the information. That would be tap water parameters, tanks contents, test readings, method used to create ammonia and water changes. More importantly, we need to have this information over time not just a single reading.
Given all of the facts posted in this thread, is is not so hard to figure out what may or may not be happening.
I will confess I did make a couple of assumptions here. one is that the tank was cycled. Even if the individual fish have a bit of a difference between how fast ammonia levels show they are suffering, they should all show some levels of ammonia poisoning. The fact that multiple fish are dying or gasping and others are just dine would indicate that ammonia etc, is not the cause. Nitrate poisoning works similarly to nitrite poisoning, but it takes a lot more nitrate than 40 ppm to produce that result.
The add in the fact that fish are dying and that more fish seem to become ill suggests something contagious is at work. At least it does for me. If we can eliminate ammonia or nitrite as the cause, then we must look elsewhere.
One last observation here. Higher levels of ammonia than .25 ppm can definitely damage gills. So if you recently cycled your tank and exposed the fish to higher ammonia levels, this could take a bit of time to show itself. One fish dies faster than another by a few days. But then all of them should be showing some signs and they are not.
When it comes to keeping fish and illness/infestations we are forced to be detectives. We cannot take out fish to a vet. Most of us lack the diagnostic knowledge and tools to do the science needed to assess the situation. All we have are behavioral clues and any symptoms we can actually see. And even then we may not have a clue.
p.s.
Apologies- I just looked at your pictures above. I suggest you read some about columnaris with the idea that you can eliminate it as the cause. Columnaris is often called mouth fungus or saddleback disease because of the white patches at the base of the dorsal fin or a white mouth which may also appear to involve fungus. However, the disease is bacterial not fungal and would require an antibiotic to remedy.