I recently got a 54L aquarium and stocked it with 4 small guppies - 1 male and 3 female. Initially they all seemed to be doing fine - eating, very active within the tank etc. The male looked great when I first got him:

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However when I woke up this morning the male looked sick and was not moving much. He has lost all colour in his caudal fin. When I got home from work he looks even worse and seems to be hiding:

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I tested my water parameters and all seemed fine. I have now put the male in a breeder box to separate him from the females so I can keep an eye on him better.

Is it possible to know what may have caused this situation and / or if it is salvageable?

UPDATE: This fish has now died as well as one of the seemingly healthy females. My tank parameters are as follows:

  • NO3- 10mg/l
  • NO2- 1 mg/l
  • GH 16 °d
  • KH 6 °d
  • pH 7.2
  • Cl2 0-0.8
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    Welcome to pets.SE :) Please write the parameters into your question. I know, the tank-experienced people here will want to know them. Also it may be important if you use a filter and other items to clean/air the water. Or in which intervalls you change water – Allerleirauh Mar 5 at 15:07
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    Nitrites (NO2-) should be undetectable in a healthy aquarium, 1 mg/L is quite high and harmful, it also suggests that nitrogen cycle hasn't fully established yet or that it was broken at some point. Do you have an access to some test for ammonia levels? I'd guess ammonia is non-zero as well, which is harmful as well. Also, chlorine levels have no business being above 0 in a healthy aquarium - please tell us, are you using some sort of water conditioner to treat tap water before adding it to aquarium? Thanks. – lila Mar 5 at 17:34

High concentration of nitrite ions (1 mg/L of NO2-) is the most concerning thing in the list of your water parameters; it is many times above the toxicity threshold, because nitrites may start to become toxic at levels as low as 0.1 mg/L. Mechanism of toxicity is that NO2- ions get absorbed into the bloodstream through gills and oxidize hemoglobin in fish's blood to methemoglobin, which cannot bind and transport oxygen like hemoglobin does. It leads to generalized hypoxia, which is a serious and potentially fatal disorder.

It has been shown that adding 10 mg/L of chloride anions for each 1 mg/L of nitrite anions protects against NO2- toxicity in most fish. Nitrite ions are absorbed via the same mechanism as chloride ions and compete with them in the context of absorption, so the rate of NO2- uptake depends on how much NO2- there is compared to Cl-.

The easiest way to increase chloride concentration in aquarium is to add salt (sodium chloride); salt is 60.7 % chloride by mass and your aquarium volume is 54 L, so to increase Cl- concentration by 10 mg/L you need to add at least 0.01 / 0.607 · 54 = 0.89 g of salt. Please note that it is the best not to use table salt, but aquarium salt from pet store. It wouldn't hurt if you add a bit more than that; in freshwater tanks, it is safe for animals as long as you don't exceed the upper limit of 0.3 % salinity (which is 300 g of salt per 100 L of water, or 162 g for your 54 L aquarium).

A precaution: while 0.3 % salinity is considered safe for freshwater animals, it could cause adverse effects in some sensitive aquatic plants; however, such (relatively) high concentration is used for preventing or treating pathogen-borne diseases in aquarium. The concentration for neutralizing nitrite toxicity is over 100 times less than this and will not hurt your plants.

The problem is, the high concentration of nitrites present in your aquarium is probably a result of a bigger underlying issue: an un-established or broken nitrogen cycle. I'd guess ammonia concentration in the aquarium is non-zero as well, which is harmful, too.

Having functional and established nitrogen cycle in aquarium is essential for the well-being of its inhabitants. More details about nitrogen cycle in aquarium could be found in this Wikipedia article and in this Q&A.

What is more, free chlorine (Cl2) levels have no business being any other value than 0 in a healthy aquarium. Free chlorine isn't produced by any process in the aquarium, so the only reasonable assumption is that it must have been introduced in the aquarium with tap water during water changes. Please always use water conditioner to treat tap water in order to make it aquarium-safe. Water conditioner doesn't have to be fancy nor expensive, but it must be able to neutralize both chlorine and chloramines.

In your question you have written that free chlorine levels are between 0 and 0.8 mg/L - I assume that the color of colorimetric test was not matching any color in the reference sheet exactly, instead it was in-between the scale, so you have posted the two bordering values that it was in-between. It might be the case that there is actually no chlorine, but the reading gave a false result. As a precaution measure I am going to assume that the test was accurate, though. Colorimetric tests might sometimes be difficult to read accurately because many different factors could distort the reading:

  • attempting to read in artificial lighting with low CRI (color rendering index); it is best to read these tests in incandescent lighting (CRI = 99+) or in natural daylight (CRI = 100), any CRI lower than that will distort the readings;
  • attempting to read while being colorblind;
  • using tests that were stored improperly or are past their expiration date;
  • using all-in-one strip tests instead of liquid tests (liquid ones are much more accurate).

Moreover, please don't confuse chloride (Cl-) with free chlorine (Cl2) - they are different forms of the same chemical element and they have fundamentally different properties. Chlorides are negatively charged ions that exist in the form of salts (most common is table salt) or in water solution and are relatively harmless; free chlorine is a highly corrosive and toxic gas that is a strong oxidizing agent.

Dechlorinating water conditioner works by sacrificing its active ingredient (a strong, water-soluble reducing agent, most often thiosulfate salt or vitamin C): it gets oxidized by free chlorine and in the process chlorine itself gets reduced to harmless chloride ions; this reaction does not spontaneously reverse on itself in the aquarium, so the dechlorination effect is permanent in this context.

However, progressively more and more tap water treatment facilities are using chloramines instead of chlorine; both ammonia and chlorine are added to the water and chloramines form in situ as a product of their reaction. In simplified terms, dechlorinating agent present in water conditioner neutralizes the "chlorine part" of chloramines, but leaves ammonia behind; a good water conditioner has a second ingredient that takes care of this fact and temporarily neutralizes ammonia as well. Unlike the case of Cl2 neutralization, ammonia is not neutralized permanently by such conditioner - however, in an aquarium with functional nitrogen cycle this is not a problem because the produced ammonia gets permanently and completely oxidized to relatively harmless nitrates (NO3-) in the nitrogen cycle before it reverts to its toxic form.

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