Ph Drop! 0.25Ppm Ammonia! Why!?

One of my neon tetra has become bloated and his scales look to be flaking off. It has always been one of my fatter neon tetra but this is much larger and it seems not to be able to keep its body aligned in the water, so I decided to do a full water test which came back as follows:

pH : unknown - doesnt register on either test, im pretty sure it is lower than 6, usually 6.4
Ammonia : 0.25 ppm (unknown reason for this too)
Nitrate : 0
Nitrite : 0

The fish in the tank are:
One common pleco, 8 serpae tetra, 2 corys, 3 neons, 6 mollies and around 20 or so young assassin snails.
It is a 40 gallon long

I havent changed anything in the tank for months (added/removed fish or decorations)
I last did a water change 6 days ago (around 30% weekly)
All of my other fish seem very healthy and none are unaccounted for

So what has happened and how do I get my pH back into range?

Can I ask whats up with the stock? One common pleco shouldn't be in anything smaller than around 400Litres at a push.
 
However, I'd test the PH again. Bearing that you are using the API masters kit you'll get a reading unless it's off the scales in which case everything in that tank would be in PH shock.
 
But in terms of the neon, It's a neon. There not hardy fish and are prone to die for no reason, I honestly gave up keeping neons just due to this fact.

Yeah ino about the pleco, it is only about 6 inches at the moment so its fine for now, I will be moving it when it is bigger.
I've only got 3 neons in there because I was sick of them dying, whether it was me or pets at home I don't know.

I tested the pH 3 times, twice with the low range and one with the high range, the high range stayed a really light brown and the low range turned a slight green/clear colour, but not dark enough to be 6. All of the fish except the neon seem completely fine.

I'll try doing a large water change and see where that gets me ;-)

My best recollection for the API pH tests kits is if one's pH is over the top on the high range or below the bottom of the low range the kits will show the highest color reading or the lowest color reading, they do not fail to show any color? In order to be able to test the pH in tanks which were in the 4.2 pH range, I had to purchase digital testers.
 
Test your KH also. It is what helps hold pH steady.
 
If your pH is indeed under 6, then ammonia is not a concern at .25 ppm unless it never drops.

I thought I would share a pic, the left is the pH of my piranha tank, I use the same water and do roughly the same %water change every week and the pH of that tank is around 6.6 the tube on the left is my community tank reading, showing now bright yellow. The tests were done at the same time.
How do I test for KH? If it's any help I use Yorkshire water tap water so it is quite soft, thanks for the replies too ;-)

Take a water sample to your local store, they should test it for you. API makes a KH kit but it is not essential to have.
 
Different gravel, rocks and wood can all have an effect on pH. Either something in the piranha tank is holding it up or something in the community is causing it to drop?

I do have quite alot of black brush algae, my sciatica has been playing up so my boyfriend has been cleaning the tank and I don't trust him to clean my glass because it is a buffer to get off so it has slowly been building up. I've just cleaned alot of it off and done a 50% water change, I have lots of bogwood, pettex roman gravel, no plants and a cave by exo terra. I will test the pH again in a while while the water settles again. I will take a sample to the pet shop though.
The piranha tank is just pets at home aquarium gravel and some fake plants.
Just confusing because the pH was fine when I tested on Sunday.

I'm having a really similar problem, and am SO worried about Dorothy.