Don't know what is causing fish to die

[Thouaweek]

I have a Juwel Rio 180 with standard bioflow filter and Nature and Day LED lighting that came with the tank. I set it up in November and cycled it for 5 weeks before adding fish gradually - 16 cardinal tetras, then six corys in two batches, then two angels, after planting densely with a number of species including amazon swords. I used a fluval pre treated substrate, overlaid with stones, gravel and sand collected from a remote river bed in the Scottish Highlands. I did the vinegar test and water tests since suggest these are not affecting water chemistry. I also maybe unwisely put an old tree stump into the tank which I also collected in an ancient woodland that appeared to be bog wood. I removed this after six weeks and replaced with an aquarium shop bought wooden piece from China, because I thought it was not helping with the algae.
Unfortunately algal growth was strong, despite LED lights being on only 7 hours a day which I quickly reduced to 5 hours a day. I used anti hair algae treatment for a month for January only as per instructions.
For the first 2 months I changed 20 litres water every week, then 40L every week since.
2 cardinals disappeared quickly, then another, and I found one looked like the angel had had a go at it, leaving 12 in total remaining and stable. One of the angels never ate anything, no flakes, sinking pellets, or odd frozen treat that the others ate, and died within a couple of weeks. I replaced this angel to have 2 again. I put in two batches of corys (3 pandas, 3 Julis). One of the Panda corys that was slightly paler than the others turned almost translucent over the first 6 weeks and died. One of the Juli corys disappeared, then 2 weeks later I found another dead Juli, still in good condition otherwise. No bloating of any of the fish. Apart from the angel and one cory, all the others looked in good condition, with no signs of disease, and no visible weight loss. But two nights ago, the remaining original angel died, which had doubled in size as they were all small when purchased. It hadn't been eating properly for four days, and started swimming within its fins behind it, hiding in the corners and at the surface of the water. It bumped into a a couple of things, I wondered if it was blind. This was a superb fish, in really good condition up to that point, so was really sad, my six year old daughter had named it "stripey". The other angel is doing well, and has also grown impressively as well.
I have had the temperature at 24 degrees centigrade.
Water testing results. After cycling, all normal - pH is between 6.4 and 6.6. Ammonia is 0, but was 0.25ppm in early January, reducing back to 0 in February. Nitrite always been zero, Nitrates have been zero up to start of March, but now at 0.25ppm. Phosphates were 0 until anti-hair algae treatments and Seachem flourish put in, and despite the water changes, have been over 10ppm since.
The plants have not done as well as expected, even the Valisneria and Amazon swords, with new leaves not growing as big as originals.
I have been replacing the top layer of the (barrier?) filter every two weeks, and replaced the biocarb filter a month ago.
My conclusions so far draw a blank after reading many threads on the forum. Because the fish have not looked unwell and there has been time between the deaths, I suspect it isn't water chemistry or a bacterial infection? I want to get back to two angels and 6 corys, but do not want to put more in if they are going to die. Am thinking of setting up another tank to put fish into while I set up the tank again.
I would be very grateful for your advice.

 

[GaryE]

That's a puzzling one.

If you have a strong concentrated light, examine each fish from the nose to the dorsal along the spine, looking for a velvet textured growth usually behind the eyes. You have ideal water for Oodinium spp, the 'velvet disease" parasite. It might explain things.

So too could the fish from the store.. Most fish deaths in the first 6 weeks after arrival are connected to shipping, although it's a hard opinion to prove. I worked in the business. Even good stores will have these problems, because they can't control how the shippers on the farms pack, and what happens on the plane and in cargo. We pull out our test kits and seek to blame ourselves, but those fish have come off an intensely stressful journey to get to you. Wintertime fish may have been chilled.

Corys should be in larger groups, of at least 6 per species, and more if you can. But that won't kill them.

Angels kill each other. If you put two males together, one will die. Since they are often very hard to sex, you can get into an endless series of tries to keep two. You can do well with 10, or with one. In between,it comes to sheer luck. It's possible you simply haven't had any.

As a personal choice, I would never use an algicide. They affect more than algae.

 

[Thouaweek]

That's a puzzling one.

If you have a strong concentrated light, examine each fish from the nose to the dorsal along the spine, looking for a velvet textured growth usually behind the eyes. You have ideal water for Oodinium spp, the 'velvet disease" parasite. It might explain things.

So too could the fish from the store.. Most fish deaths in the first 6 weeks after arrival are connected to shipping, although it's a hard opinion to prove. I worked in the business. Even good stores will have these problems, because they can't control how the shippers on the farms pack, and what happens on the plane and in cargo. We pull out our test kits and seek to blame ourselves, but those fish have come off an intensely stressful journey to get to you. Wintertime fish may have been chilled.

Corys should be in larger groups, of at least 6 per species, and more if you can. But that won't kill them.

Angels kill each other. If you put two males together, one will die. Since they are often very hard to sex, you can get into an endless series of tries to keep two. You can do well with 10, or with one. In between,it comes to sheer luck. It's possible you simply haven't had any.

As a personal choice, I would never use an algicide. They affect more than algae.

Thank you Gary, I will check for the Oodinium. The angels got on well together, I think the golden one spawned at one point, and the one that died was the dominant one.