Fish Dying And I Don't Know Why

Hi, this is my first time posting on this forum.

I am fairly new to the hobby of fish keeping, and I started setting up and cycling my first fish tank around 5/6 months ago. I waited for the tank to cycle and the gradually started adding fish, which all went extremely well. I lost a couple of fish, one guppy as it got trapped behind the filter, and also 2 tetras due to swim bladder disease, but other than that they all seemed very healthy. Over time I gradually upgraded the equipment, installing an external filter, making sure I left in the old filter for a month for the bacteria to start growing in the new filter, and most recently an in-line heater as well, which I installed maybe a month ago. A month and a half ago, or so, I noticed a white spot on the bottom jaw of one of the Zebra Danios and I treated with Tetra LifeGuard and ,although a couple of my neon tetras also developed white spots on their lower jaws, it cleared up on the Danio and has not worsened on the tetras, so I am assuming they will also disappear some point soon. NOTE: none of the fish with white spots on their jaws died, in fact they seem very happy.
 
Recently however, over the past month, fish have started dying seemingly randomly. Over the month I have lost 3 Bristlenose Plecos, 2 Zebra Danios, 4 Guppies (two in the last week), and 1 Galaxy Rasbora. I did not see any sign that the Plecos were stressed or sick, although I did notice that one of the Danios, the Guppy and the Galaxy Rasbora were looking very skinny, as if they were underfed, although all the other fish in the tank seem extremely happy, even somewhat plump, so I thought perhaps something causing loss of appetite?
NOTE: I feed them tropical fish micro granules one day and then frozen bloodworms the next, I alternate every day and fast them on a monday

Below I will list my equipment, fish currently in the tank, and all of the water parameters I have measured. The fish deaths only really started happening 1/2 months ago, and have been consistent since then, but I cannot think of anything that occurred that could be causing them, the water parameters have not changed and I haven't added any new fish. Any help would be really appreciated as it is heartbreaking to be losing fish, and its extremely frustrating as I feel I have tried my best and don't understand what could be wrong.

Equipment:
- 125L Fluval Roma Tank
- Fluval 306 External Filter (1150LPH flow rate)
- Hydor 200Watt in-line heater
- Tetra APS 150 air pump
- Arcadia LED lights
- Flow bar pointed towards surface to increase surface agitation for oxygenation of the water

Fish in tank:
- 3 Yoyo Loaches
- 3 Scissortail Rasboras
- 2 Zebra Danios
- 0 Guppies (the last 2 died this week)
- 6 Neon Tetras
- 3 Galaxy Rasboras

Water Parameters:
- Temperature: 24°C (very stable since addition of in-line heater)
- pH: 7.7 (I dont know if this is a bit high? but it is extremely stable so I have left it where it is)
- Nh3 (Ammonia): 0 ppm
- NO3 (Nitrate): 25mg/l (I change 20% of the water weekly)
- NO2 (Nitrite): 0 mg/l
- GH: 8°d
- KH: 6°d
- Cl2 (Chlorine): 0 mg/l

Thanks Sue,

Yeah I think the water around here tends not to be too hard, although I wasn't aware that this could be a problem. As far as I am aware it hasn't risen or fallen since I set up the tank, and although I have been told by multiple people that guppies have a tendency to be weak due to the breeding, the reason for my concern was the other fish, especially Bristlenose Plecs which I was told were fairly hardy, dying and the fact that they all seemed happy and there were no deaths for the first 4/5 months of the tank running.
In regards to the Yoyo loaches (Pakistani loaches), I have not noticed any aggression from them, they seem to have settled in nicely now and mostly stay around the bottom and tend not to bother the other fish, even during feeding time where the eat last as the food sinks. Visually they also look the healthiest and most confident, although the others also look confident. I also have the resources to move the Yoyo's and the Scisortails when they get too large, although at the moment they are only like 4/5cm so I don't think that's too big for the tank? although I could be wrong.
And in regards to the white stringy poo, I have spent some time watching them and looking closely and I dont see anything that would fit that description, although their poos can sometimes be red, but I believe that is due to the colouring in the food?

Thanks,
Nye

Sorry I can't really help more... just wanted to say I also suffered early neon deaths and the others have lasted years. I just put it down to there being more and less genetically lucky fish...