Fish occasionally dying :(

Hello, I keep having a fish death on average one every month. I dont understand why as all the fish are in the same tank and all look fine, then only one of them will start looking really unwell and in a few days will die. It only happens to them one at a time. I have 3 guppies, 2 cherry barbs, 6 Neon tetras and 2 glass phantom cat fish. It is mainly the guppies dying. In the last couple of days I noticed one of them is sick and I;m sure will die in a day or two. This picture is how he looks at the moment - tail is closed up instead of a wide fan as it has been, his body looks bent and he stands quite vertical in the water, all these things are completely different from how he has been in the several months that I have had him. I'm not sure what is causing this. My questions are is there anything that can be done to help him or is it too late for him. Also what is the life expectancy of a male guppy when bought from the pet shop? They all seem to be doing well for a few months and then they get unwell like this and die.
I do water change every week to keep up with the Nitrates. Otherwise Amonia is 0, Nitrites = 0, Nitrates around 10 - 20 and has never been any lower. PH around 7.5.
One thing that is making me uneasy is the water temp which is set to 24 deg. But during the summer when we have been away for a whole day, it has reached 30 in the tank. I wasn't about to run the air cond in the house just for the fish tank. I doubt anyone does that. Can anyone see anything obvious by looking at the picture? I had 10 guppies, now I only got 3 left and soon they will be two. I have also lost 2 neon tetras and one cherry barb. Again same thing one of the cherry barbs died, while the other two seem perfectly fine and are doing quite well at the moment.
Thanks for any advise

Hi can't see anything from the pic but as you said it is mainly the guppies do you have all males as they will show off to each other and pick at each other. It may be an injury causing the problems 2-3 females per male is a good ratio although will result in baby guppies.

That said it is hard to be sure but if your water parameters are as you say I am inclined to think injury rather than anything else. If possible I would suggest moving the sick fish to a hospital tank or dividing part of the current tank to isolate it away from the others for further observation.

Assuming the water parameters posted are correct it shouldn't be the water causing problems although I would try to get the nitrate down more have you tested your tap water.

Also is the fish eating ok, there is a possibility of swim bladder problem but this may be stress related

Also, Cherry barbs need a group of at least 6. The glass catfish need this too. This may be causing stress and aggression between the barbs and the other fish.

Unless you're seeing signs of aggression, I think it may be some kind of water issue. You should test your tap water. Is it treated (chlorine/chloramine)? What conditioner do you use? The tank looks planted. What additives (ferts) are you using? What is the tank size? Filter? Food and feeding? ...???

Do you have soft or hard water?

Thanks for all the advise guys!

Thebadwolf, I only have male guppies but never seen them fight or nip each other.
The Nitrates have been around 10 as soon as the tank got properly cycled.
When I test tap water - Nitrates are 0. For a while I would replace 1/3 of the water and could not see
any reduction in nitrates. More recently I have been replacing around 3/4 of the water and the nitrates have
been reducing to 5 for a few days before climbing again.

King puff, thanks, may be this has something to do with it. I have not seen any aggression and I do stare
at them for long time. But who knows what happens at night. The glass phantoms are a lot more active at
night and are bigger than the other guys.When I bought them I was told they will get along with the others
and the pet shop had glass phantoms with other smaller fish in the same tank.

AbbeysDad, when I do water change I add tap water conditioner, see in the picture. To this point - is it
possible to over doze on this stuff. Once again when I asked in the pet shop they said a convincing no. But
this dosage is 1ml per 60L of water and I am adding 1ml per 10 - 15L. Simply because I can't measure any smaller amount and because I was reassured that I can't over doze it. I was also thinking if it is the water why I never get 2 or more fish sick at the same time?
The tank is not planted. It only has a plastic plant and another ornament - no live plants or ferts.
Tank size is 30 L
Food is as in the picture below.

NickAu I dont know and have not checked if the water is soft or hard. Seeing you are from Australia,
may be you will know - it's Adelaide tap water. Is there anything I can do to improve the water other than add the conditioner in the pic?

Thanks again for all the help.

Dechlorinators should not be overdosed but used at the dose rate given on the pack.

I use the same dechlorinator as you and I refill the tank using a bucket. The dose rate for this is 1 drop per 1 US gallon or in metric that's 1 drop per 3.8 litres. If you use a bucket to refill your tank I suggest you use a dropper/pipette to measure the dechlorinator.
If you change 10 to 15 litres, you need 3 drops for 11.4 litres, or 4 drops for 15.2 litres.
I just measured 7.6 litres into my 8 litre bucket and marked the level; now I run in water to my mark and add 2 drops dechlorinator.

Instead of using a pipette to measure drops you could buy a 1 fl oz/30 ml bottle which has a dropper lid not a cup lid, then when it's empty buy the larger bottles (which work out cheaper per ml) and refill the small bottle. That's what I've done.

I forgot to mention that if you have all these fish in a 30 litre tank, it is overstocked, and poorly stocked.
Cherry barbs and neon tetras need a tank at least 60 x 30 cm - a 60 litre tank minimum.
Glass catfish need a tank at least 90 x 30 cm.
These fish are all shoaling fish and should be kept in groups of at least 6.