How to tell if Betta's infected?

Hi everyone!

TLDR at the bottom;

I've come here looking for help working out what's going on with my fish - over the course of the last day I've had multiple tetra's die and I'm worried about my Betta now . I've owned a tank of fish for around a year now with a Betta and 10 tetras without any major issues, but a few days ago I noticed one of the tetras had gotten sucked into the water filter and died there (presumably that's the order anyway). I foolishly waited several days before getting around to replacing the water filter, along with replacing most of the original water.

I thought everything was fine again, but today I found all the fish seemingly gasping for air at the top of the tank - and when I came over and most had dispersed, one of the tetras was still there, kind of upside down vertical. I quickly isolated him into a bowl filled with warm water (I don't have a second tank) and added the usual water ager (Marine Master Water Ager), but he died within half an hour. Thinking it could be contaminated water from the dead fish (it was also kinda murky already) I completely replaced all the water, and scrubbed everything in there with detergent (of course rinsing it all for ages before putting it back).

Since then more have shown the same symptoms and died - even after I tried treating them with Wardley Promethyasul, and I've now completely removed the tetras into their own bowl (the Betta hasn't shown any symptoms, yet anyway).

One of the tetras started showing symptoms and having no new ideas I tried adding the same antibacterial stuff...but more - this time however he's survived for a while longer and is showing additional issues. His back - and now his torso has turned black while his underside has discolored; interestingly sometimes he manages to get the right way round and swim for a bit, only to eventually flip back around and stop moving. All the remaining tetras are also now almost completely still and are just twitching, every so often one might have a panic attack and race for a bit however. A single tetra in the non symptom bowl also has a curved back, but I'm not sure if that's related or a genetic issue. I've also noticed none of the tetras are eating, nor have I seen any poo. There's also a very faint red stripe down the sides of the fish, starting from the tail and fading towards the head.

TLDR;
Tetras dying from something that causes them to turn black with discolored belly, possibly have a curved back (only shown on one), swim upside down, gasp for air, not eat or poo, have a very faint red stripe at their tail end, and twitch in weird ways - it could be caused by having had a dead fish in the tank for a few days, maybe multiple recent water changes, or something else entirely. I add Marine Master Water Ager to all the water I use to remove chlorine, and have also tried treating the fish with Wardley Promethyasul to not much avail.

Sorry for the long post but I'm hoping some of the details there are helpful. I've got a feeling the tetras probably won't make it, but I still really want to figure out what the issue is to see if it'll effect my Betta. So far my searches have turned up Neon Tetra Disease and piscine tuberculosis, which both suck pretty badly, but they also don't seem to fit all the symptoms quite right so I'm hoping it's something else. I'd be really grateful if anyone could help!

The non symptom tetras (all the stuff at the bottom is food, I realised they weren't eating and kinda overdid it...you can see the one with the curved back - it doesn't curve down at all):


The still barely alive tetra (same here foodwise):

The tank (water filter, heater):


Thanks!

first thing to do is do a test for ph, ammonia, nitrites and nitrates.... also check temperature. if you don't have a test kit, bring a sample of the water to the local fish store and they probably will test it for you

what size is your tank...appears to be a very small tank..you may be overcrowding the fish which can cause ammonia and nitrite spikes

also what is the hardness level of the water...this is available from your local water supply company or you can do a test for it

overfeeding can cause ammonia spikes if tank is not fully cycled...

it's possible they died from a common disease among neon tetras called neon tetra disease...a curved back is one of the symptoms

Thanks! Looks like I was overreacting regarding any diseases . I took samples of the water from before the complete water reset and it turns out the nitrate levels were really high (I think I hadn't been cycling the water enough ) and that was likely what was causing the problems. I'm still a little concerned just due to the curved back tetra and some of the timing - I would have hoped isolating the tetras would have fixed any nitrate issues but I guess they were already damaged inside .

All the tetras in the non symptom bowl have survived however and I've reintroduced them to the main tank, they seem much happier . I'll be keeping an eye on the curved tetra, hopefully it's a genetic issue (we've had him for a year now so it's unlikely it's NTD), and I've added some artificial beneficial bacteria to get the new colony growing quickly. Throughout the entire thing my Betta has just been shrugging it all off haha, so yay! Thanks for your help .