A Sad Little Green Tiger Barb

Nitrates are nitric acid in solution and will make the water acid (and nitrites are nitrous acid). That's what happened during my fishless cycle, though admittedly I did start with a lower KH than in this case.

Any solution of nitrate in water will have both NO₃^- and H⁺ ions, which can be regarded as nitric acid. Carbonate raises pH because carbonic acid is a weak acid; that is, it dissociates in water to a much smaller degree than a strong acid like nitric. A high concentration of H⁺ in the water pushes the equilibrium over toward carbonic acid, and H⁺ is removed from the water, raising the pH

Actually, what I wrote above about carbonates is quite likely rubbish  I just realised what I wrote while making the beds. It would be true of other weak acids, but

CO₃^2- + 2H⁺ -> H₂O + CO₂

or in other words, a carbonate plus acid in the presence of water gives water and carbon dioxide. That's how the carbonate (and bicarbonate) ie KH is used up.
This is why you find bicarbonate of soda in the home baking section. It is mixed with an acid such as tartaric acid in both baking powder and self raising flour. As soon as moisture in the form of eggs and/or milk is added to a cake mixture, the bicarb and acid react creating bubbles of carbon dioxide. The cake mixture is then heated, setting the mixture and trapping the bubbles. It is why cake mixture should be put in the oven asap or the resulting cake will be flat.


As for the pH drop when nitrate is formed - biological entities are at work here. It is not likely to be a nice plain chemical reaction. Biological entities use enzymes and all sorts of intermediates to carry out reactions that are not easy to do in a test tube. this is how we convert sugar into carbon dioxide, water and energy. Not exactly a straight forward a -> b reaction. It would not surprise me if it turns out that the bacteria do all sorts of complicted things to turn ammonia into nitrite then into nitrate, and there would likely be all sorts of other rections going on inside the bacterial cells as well, the products of which could well end up outside the cell in the water.
Whatever actually happens, the direct result of the nitrogen cycle is nitrate in the water and a lot more hydrogen ions - ie a lower pH.






I would have preferred to study biochemistry rather than chemistry but I had a major disadvantage - I just couldn't memorise all those metabolic pathways. So my degree is in chemistry with biochemistry as my auxilliary subject. At the uni I went to back in the 1970s, we studied 3 subjects in the first year, then decided on the degree subject after the first year exams, studying 2 subjects in a 2:1 ratio in the second year and just one subject in the third year. I did chemistry, biochemistry and zoology; chemistry and biochemistry; chemistry. My husband did chemistry, physics and maths; chemistry and physics; chemistry. Yes, we did meet at uni 

My CSE in chemistry just isn't up to all this stuff.  It was 40 years ago!  I started out keeping fish, I didn't want to become a chemist or a lighting technician or a photographer or an electrician or a structural engineer or a gardener or a web designer or ............

I just want to keep fish!

Haha yes I'm with water watcher as a Psychology BSc myself ha

That's good news.... but I do stress the importance of being able to test for ammonia at this stage in your fishes life as this will determine how much water you need to change to keep the level below 0.25ppm. Above this level then it can start to damage their gills which will stress them and make them more prone to disease.

Us BSc's will always stick together!!

Water checked at the fish shop all levels seem ok. Another member of staff suggested that's perhaps I had two males and one female, which may have caused the problem. I have a male red wag barb, not sure about the guppy. When I get some more fish ill make sure it's female though to balance it out.

The though has spent a lot of today sitting on top of the filter which is also right next to the heater, it's not gulping air or anything. It's just sat there!!

Any suggestions?

It must be female - like my better half and always cold...

Sorry, no idea

To continue from Colin's post -
Oncce you have identified the sex of your platies and guppies, you need at least two females for every male. Livebearer males will chase females constantly. You need more females than males so that each female gets time out while the males chase other females.
And male platies will chase female guppies, and vice versa.

If you have all males or all females that's fine. It's just if you choose to have mixed sexes that you need at least twice as a mny females as males. Three times more is even better.

Bought an ammonia testing kit and some more activated carbon filter. I've heard when I change the filter to cut it in half and only change half at a time. Is that right?

This image has 5 and 6 the wrong way round I'm afraid.  The fins nearest the gills are the pectoral fins, the fins a little further back are the Pelvic or Ventral fins.