Established Tank - Issues For A Long Time

Test kits are not always absolutely reliable. If you know your water supplier, they usually have a web site where you can enter your postcode and get the parameters for your supply. It's worth checking, just in case, they have much more sophisticated test equipment. I'm always a bit suspicious when anything other than ammonia or nitrite come out as zero unless it's pure rain water.

The symptoms of your problem sound like old tank syndrome to me.  Your old fish aren't carriers, they're just used to the water parameters as they've grown with the tank over time.  The new fish come straight from the fish store and aren't used to your water parameters and die.

The problem is your water parameters as far as the pH going up doesn't match an expected pH drop with old tank syndrome.  The low kH is a classic indicator though.  The good news is there's a very easy fix.  Add baking soda gradually to raise the kH to around 3-4 kH.  Gradual is important as your existing fish will be killed if you try to go to 3-4 kH in one go.

Modified:
Oops, I need to add that adding baking soda will raise the overall pH of the aquarium!  If you don't want to raise pH try and get hold of some potassium carbonate (which shouldn't be hard as they're used in planted tanks).

If the root cause is old tank syndrome when the kH is up give the aquarium a deep clean.  Clean the insides of the glass and get as much gunk out as possible.  If the root cause is low kH in your tap water you'll need to add the source of carbonates to the water your adding at the same ppm.  So make sure to do some measuring as you go.  I would recommend going the potassium carbonate path as sodium is basically salt and you really don't want to get too much of that in your tank.

Personally I am not a big fan of adding bicarbonate of soda to a tank with fish in. Apart from the Rift Lakes, no fresh water contains much in the way of sodium, or even potassium.

Have you increased your water changes form more than 17% once every week or two? In the absence of well growing, fast growing plants, there will be a lot of chemicals building up in the tank, which is old tank syndrome. Water changes will also replenish the carbonates in your tank.

Do you know the KH of your tap water? It could be worth taking a sample of tap water to a fish shop and get them to test both GH and KH, then we have a base line for your tank. If you have at least KH 3 in your tap water you should be OK with 40% weekly water changes. This is how I keep my KH topped up.

The thing is that we can't test for more than a handful of things. There are countless chemicals excreted/secreted by the fish which we can't test for. Many years ago I used to work in a hospital chemical pathology lab and you'd be amazed at what we tested urine for. The fish equivalent of all those things, plus countless more, end up in the tank water. When insufficient water changes are done, these things build up and as they do the fish acclimatise to them. This is what old tank syndrome is. When new fish are put in the tank they are not acclimatised to them and the sudden move to a tank containing a lot of chemicals damages the new fish.

Old tank syndrome can also affect the existing fish if the fish keeper suddenly does a big water change. This is why when a tank suffers from OTS, small frequent water changes must be done initially to gradually dilute all those chemicals and allow the fish to acclimatise to lower levels.

There are a couple of useful articles on the PFK site that I found a while back which relate to old tank syndrome.

http://www.practicalfishkeeping.co.uk/features/articles/2016/6/23/how-to-avoid-old-tank-syndrome?rq=old%20tank%20syndrome
http://www.practicalfishkeeping.co.uk/features/articles/wise-up-on-wipe-outs?rq=old%20tank%20syndrome

@Lee2Gould, did you ever find a solution? I was searching for something else and found this conversation. Your problem sounds an awful lot like the issues I had when I set up my tank. I have fairly neutral pH, soft water and most importantly very low nitrates.

I also have a planted tank. You said you had plants, but they weren't thriving. How many plants do you have and what type of plants.

I eventually discovered that the issues included low calcium (not hardness, which measures carbonate part of the calcium carbonate), which both fish and plants need. Fast growing stem plants and faster swimming fish need more calcium than slow growing /swimming plants and fish.

The ratio of phosphate to nitrates is also really important. The phosphate comes from the fish food, so you need to make sure that you have enough nitrates in your tank to enable the plants to use up the phosphate.

When considering terrestrial plants, fertilisers always talk about NPK, which is nitrogen, phosphate and potassium. Aquatic plants are the same, and it is the balance between these three macro nutrients that optimises growth and water conditions.

I finally got my tank into good condition by adding calcium nitrate and potassium nitrate salts at water change (as well as dosing liquid micro nutrient fertilisers). Once I'd sorted the plants, the fish started to thrive and as I increased the number of fish in my tank, I was able to reduce the additional nitrates.

Something else to note is that a lot of easy to care for starter plants have high demand for nitrates - this is good for most people who have to keep nitrates down with water changes. Less good for those of us that have water with low nitrates. My Amazon sword died completely when my tank got neglected and I stopped adding nitrates.

Hope it isn't too late for your tank.
Helen

It seems that nitrate is plants' second choice for fertiliser - they prefer ammonia. This is because a plant has to turn nitrate into ammonia inside itself to utilise nitrate and this uses energy.
With a fully stocked tank the fish should provide enough ammonia for the plants to use. In fact, in a fully stocked tank with a lot of plants ('lot' is the important word here), we don't actually need a filter as the plants would use all the ammonia made by the fish. Lightly stocked, heavily planted tanks would need extra nitrogen fertiliser though.
Aquarium plant nitrogen fertilisers would be better for the plants if they contained ammonium salts rather than nitrate, but the addition of a dose of ammonium salts would not be good for the fish so they have to contain nitrate instead.

Your tank is not fully stocked at the moment, if I recall - there is room for more fish. The numbers of fish you do have probably won't make enough ammonia for all the plants you have, even while you have fewer plants during the substrate change. So yes, you will need to add some form of nitrogen for the plants.
The fact that your tank nitrate is more or less the same as your tap nitrate suggests that the plants are indeed using all the available ammonia from the fish. Plants don't turn ammonia into nitrite or nitrate like the filter bacteria do.

I have slow growing, low light plants of the kind that are attached to decor, and I too have very low tap nitrate. But my plants seem to thrive without the addition of nitrate. I think this is because my fish are stocked to the max, and probably overstocked  So lots of ammonia for my plants.
I did turn my filter off one evening in spring to straighten it (I had done a water change and the filter was on a tilt) and only realised I'd forgotten to turn in back on the following evening. After a panic, and tests for ammonia and nitrite I found zero of both. The plants and the bacteria in the biofilm all over the tank had used all the ammonia made by my fish in the previous 24 hours.
But I don't recommend anyone turns their filter off to test their plant ammonia uptake