Lost two fish even though everything looks fine

Hi
Some time ago I set up a new tank. The capacity is 126 litres (about 30 gallons). It stood for a long time without any inhabitants. It's a typical fish tank with basic plants, low light LEDs, some gravel and sand. There is a canister filter and a sponge one which aerates the water with bubbles. After I felt like it's ready I started trasfering fish from my previous smaller tank. In total I relocated 13 guppies (P. wingeI and P. reticulata mixed, most of them small in size), one pleco that's about 1 year old and one Amano shrimp. This might look like a pretty weird mix but they lived together fine. Anyway, after moving they looked stressed but eventually all got accustomed to new conditions. After about a week I decided to buy some more fish because the big tank felt empty. I purchased 4 pearl gouramis, 5 Indian glass fish and 6 cory catfish. Acclimated them by first submerging the bag, and then putting them in a bucket and letting aquarium water mix with the bag water by dripping into the bucket. Finally moved the fish with a net. They all seemed fine and pretty quickly got used to new surroundings. Somewhere along the way one of the guppies died but it was just one and I thought "well it happens". Issues started about two weeks later, just when I was about to do the first water change. On friday I noticed one of the Indian glass fish being tossed around by the water current when it came near the filter, it barely could move on its own and had its spine slightly bent sideways. I figured out maybe it's bloated because of overeating, even though it didn't look bloated but nothing else came to my mind. Left it in peace thinking I can't really help it as it shows no other signs of disease like ich or rot. Temporarily stopped feeding them for a day or one and a half. One of the other 4 Indian glass fish was behaving kind of shy but everything else seemed to be alright. When I woke up on Saturday morning, both of these fish were dead. One was laying flat on the bottom of the tank and looked like it was already eaten inside (maybe done by the shrimp after it died). Another one was bent sideways and floating in the corner of the tank. I took them both out and disposed of them. Since then, nothing else happened (it's now Monday morning in my country). After getting the dead fish out, I replaced about 15 litres of water. Cleaned the plastic lid because there was leftover food that stuck to it and had mold over it (fish couldn't access it though as it was way above water surface). Did water quality tests and got the following:

pH: 7,8-8 (somewhere between)
Ammonia NH3: 0
Nitrities NO2: 0
Nitrates NO3: 10-20 mg/l
General hardness GH: between 1-2 German degrees
Carbonate hardness KH: 13 German degrees

This is before the water change (tested from the water I siphoned out of the tank). I live in a rural area and we have really hard water, before I started this tank we had a softening and purifying device installed. Since then the water got significantly softer as it doesn't leave those white stains etc. in the sinks. Tap water (softened and purified) parameters are:

pH: 7,4
General hardness GH: 0-1 German degrees
Carbonate hardness KH: 13 German degrees

Temperature in the tank is right now about 27 deg Celsius (analog thermometer). There is no heater at all.

So, that's about it. I tried to explain everything I did. Can you spot a mistake? Was the fish dying just bad luck? Thanks for any responses.

 

What were you feeding them? How are the other 2 glass fish? It very likely could have been bad luck or something from the pet store. Closely monitor your other fish for any similar problems

 

There's 3 glass fish left. They look fine, behaving normally for now. I feed them flakes and dry daphnia. Also adding some wafers and small sinking tablets for pleco and cories. What worries me is that two died at once, just sounds like it's not very probable to happen without a reason. Going to keep checking on all other fish too. Thanks for a reply.

EDIT: Could this have something to do with the weird hardness of water? (low GH, high KH) Doesn't seem so because others are fine, but anyway...

 

Its always a possibility... I was concerned the glass fish weren't eating the flakes, just make sure they are eating next time you feed them

 

They are definitely swimming up to get the food. They catch some of the flakes. Sometimes they spit them out, but sometimes they swallow the flakes. Next feeding I will try to make the flake pieces smaller. It appears they are most interested in the food when it's either sinking but not on the bottom yet or still on the surface.

 

By spine bent, how was it bent specifically?

Was the body curled into a c shape, or was the spine itself bent up like a rollercoaster dip for lack of better descriptor?

What type of filter do you have?

I'm wondering if you have a very powerful intake filter with no cover and the fish for stuck on it? Since they are sort of fragile. Could also be just a bad stock, given that they are wild caught to my knowledge and you rainbows and other fish are fine.

 

What I meant is the entire body was curled, more like the c shape. When both of the fish were dead, only one looked curled, the other one was like eaten inside already (?) and laying flat. I think the filter isn't the problem. I have a canister type Aquael MinikanI and a powerhead-with-sponge-style Aquael Pat Mini. Both have sponges on the intake and I made sure no part of the inlet is accesible to the fish.