Leaving all fish behind currently!

[NoFishLeftBehind]

Hi. I'm fairly new to fishkeeping (2years). I'm stuck with this one and don't know what I've done wrong. Last week I found a guppy dead with it's stomach eaten. Two days later another. Then a dwarf gourami that appeared healthy and was acting normal literally an hour before, then a Betta imbellis female. All female, although I think that's a coincidence. I have 3 fish swimming on their side now. On Sunday I did around a 70% water change, even though it was testing perfect on my nt labs kit, due to the deaths. Today I've checked, no ammonia, possibly 0.25 nitrite, but it's hard to tell as it looks clear, but up to 40 nitrate (it's over 20, but not quite dark enough to be 40). What on earth have I done to my tank? It's full of plants, I've spent around 150 on plants alone, plus some stems.
I have guppies, platys, gourami, bristlenose, wrestling fish and khuli loach in a 240litre tank. 98% stocked on aqadvisor, and it's over filtered, I think it said 280% for filtration. Please help.

 

[Aiken Drum]

Did the water change have any effect on the surviving fishes behavior?

What did you do in the timeframe before the fish started dying? Anything new gone into the aquarium? Fish, Aquascape etc? Did you do a water change? Any new products in use?

How long has this aquarium been set up, and how long have you had the fish in question?

Whats an normal water change schedule?

Have you been testing water parameters frequently, or just recently because you have noticed an issue? If you have been testing the water frequently, how do historic parameters compare with your latest set of water tests?

Might something have got in the water? Any aerosols or detergents used in the vicinity? Might you have forgotten to use a water conditioner when you did a water change?

40ppm is on the high side, but not at a level where it will kill fish in short time frames. You might find it will lower health levels generally and shorten life spans, so if 40ppm is a normal level it could just be time catching up with you.

Most commonly kept aquarium plants are low nutrient demand and won't have much of a noticable effect on nitrate levels, especially on a fully stocked aquarium. What plants do you keep?

Can you post a photo of your aquarium and the effected fish? Maybe do a video, upload it somewhere like YouTube and post a link,