In short (to answer your question), ammonia poisoning killed your fish.
For the long answer see below.
I've been given a Fluval Edge and had it up and running for about 2 months.
^^-- What did you do to get it 'up and running'? Just fill with water and dechlor and leaving it running? Add 'bacteria in a bottle'? Add ammonia? Fish food? Dead prawns etc?
If you simply filled with water and dechlor (even if you also added bacteria in a bottle), this 2 month period will have done nothing to ready your tank for fish.
I got two water treatments with it 1-tap water treatment and 2-"good" bacteria additive.
^^
Tap water treatment is dechlorinator to remove chlorine, chloramine and heavy metals from the water. I'd personally recommend seachem prime, but the one you have will probably be doing the job fine so it's not going to cause a problem.
Bacteria in a bottle is often useless. Even if it does contain the correct bacteria (which not all do), they will almost always be dead before they reach your tank.
Unless the bacteria was a proven to work make that has a clear use by date and had been stored in a fridge then it's definitely useless.
I added 5 guppys and and 5 neon tetras. I lost one tetra and a few guppys, which is to be expected at the beggining.
^^
This is a VERY old way of thinking. There is no reason your fish should ever die once introduced to the tank for the first time.
You need to learn about 'cycling', please see my signature for links. You want to read 'what is cycling', 'fishless cycling', and 'fish-in cycling'.
Your tank (by the sounds of it) hadn't been cycled at all Ie. the bacteria needed to convert fish waste, ammonia (which is toxic), into nitrites(which are also toxic) and then those nitrites into nitrates(which are safe), weren't present in your tank (more precisely in your filter).
Fishless cycling is a process of artificially feeding the bacteria ammonia to carry out the nitrogen cycle.
Fish in cycling is using the fishes waste to carry out the nitrogen cycle. Fish in means instant fish, but also mean large (often twice daily for the first couple of weeks) water changes.
Last friday I did a 50% water change and filter clean (cleaned in tank water).
^^ Hopefully you now understand you were in a fish in cycling situation so should've been doing large water changes (80% or more isn't uncommon) twice daily (or however often your test kit depicted to keep the ammonia and nitrite below 0.25ppm).
The ammonia levels will have built up to very high levels, this causes burning of the gills and eventually the burning gets bad enough that the fish cannot get enough oxygen from the water.
Did you start seeing gasping by any chance?
On saturday i bought a small Bala, 2 Clowns and 2 Kissing Gouramis.
You do realise you have a 27l tank right?
Bala sharks grow to 40cm and should technically be kept in shoals. Is the fluval edge even 40cm long? Surely you did the math there right?
Clowns? I presume clown loaches? These guys grow slowly but grow up to 30cm...they're also really really heavy bodied. You're talking nothing short of 3ft (150l) as a grow out tank really... with the neccesity to upgrade in the future to allow them to grow properly. Clearly unsuitable for a 27l fluval edge.
Kissing gouramis 20-30cm and are quite heavy bodied. Minimum 200l tank really just for youngster due to tempremant and the need to have room to grow.
Really with the fluval edge you're pretty much restricted to fish that grow to around 4cm max and are slim bodied. The neons and guppies (if they were male) were a good choice to be honest. However 5 neons and 5 male guppies will bring the tank to it's full stocking capacity! And at the very least you'd have to wait a good 6-12 months until all the fish were definitely fully grown, then judge whether it would be suitable to add any more fish based on what your weekly water test results were. Ie. if your nitrates were climbing to quickly then you wouldn't want to add more fish.
I immediately bought a 6-in-1 test kit and everything was fine.
^^ 6 in 1 test kits... paper strips right? These are grossly in accurate and don't test for ammonia. As you were only at the very beginning of the cycling process the ammonia would've been sky high but there would be no nitrites or nitrates present (with the exception of what was already present in your tap water).
So where you see 'perfect results', if you'd tested for ammonia you'd have found they were probably unbelievably high.
I personally use the API Freshwater master test kit. It is liquid based (much more accurate), and also comes with everything you need. Ammonia test, nitrite test, nitrate test, ph and high ph tests.
They can be picked up relatively cheaply on ebay, much much better to have your own than rely on shops. But I'd rather send people to shops (shops almost always used liquid kits) than you use those strips.
Sooooooooo... does your head hurt yet? lol.
I'm really sorry to hear of your losses, especially if you're anything like me cause you probably feel a bit guilty now. Don't completely blame yourself though, although you should've researched beforehand, mistake to happen. Plus I personally believe shops should have slightly more duty of care and help people set up tanks correctly instead of just selling you a product (fish).
You're now at a cross roads... you could:
Keep the edge and fishless cycle it.
Keep the edge and fish in cycle it (to fish in cycle and edge I wouldn't use more than 1 male guppy).
Get a bigger tank a fishless cycle it.
Get a bigger tank and fish in cycle it.
The only reason I thought I'd throw 'bigger tank' out there is because the edge really limits your stocking possibilities due to it's size. However if you like it then it's just a sacrifice you'll have to make, and there are plenty of 'nano' fish around (2-4cm).
Personally I'm not a fan of the edge, but that's just my opinion. I find them rather overpriced for what they are, very restrictive both scaping and stocking wise, plus they're harder to maintain than your average tank.
