Guppies Dying

Hi all. Recently bought a 100l fish tank and am having problems with my guppies.
Tank size: 100 litres
Ph: 7.2
Ammonia: 0.5mg l
Nitrate: unknown
Nitrite: unknown
Kh: unknown
Gh: forget the number but its fine
Tank temp: 26c
Fish symptoms: all seem happy and unaggressive, possibly some missing tail and possibly fin rot but I can't tell. Two fish died last night a one a few days ago. All seem to be missing a lot of tail but no other fish seem to be missing a lot if any and fish were not missing any the night before.
Water changes: weekly but since there is slightly high ammonia we change it every few days for the last week or so and we've also done 50% after each death. Normally 25l change
Chemicals: water conditioner, crystals to increase gh, both recommended by pet shop. Had thoroughly washed old bourbon bottle in for a few days a week or two ago before being told to remove it. Also have gravel, live plants and river rock in the tank.
Inhabitants: one large bristlenose, a few platys, a few neons and a fair few guppies. Total of 20 fish not sure how many of each they are hard to count and my girlfriend bought the last half of them.
Everything is a recent addition to the tank since its about a month old or less. Water was tested by pet shop two days ago and everything tested fine except the small amount of ammonia which shouldn't be a problem and we are working on. I will upload pics soon but I can't see any real symptoms but I dint know what to look for really.
Thanks for any help.

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Ok, most likely you have not cycled your tank. when you do not cycle your tank it'll kill or harm your fish badly. do a fish in cycle (punch it into the search forum and you'll find a whole thing on it (very helpful))
you have fin rot in your one pic, and most likely ammonia and nitrite or nitrate (always get the two mixed up sorry) poisoning. 
 
do a large water change with treated water and get on that fish in cycle. do not buy and more fish until you are able to have your water stats perfect for around one month, and then you'll need to know what kind of fish you can keep with what etc etc.

Ok so I am new to this hobby but have done some research and gotten advice. According to a mate who had a salt setup, algae is a sign that the nitrogen cycle has started because I asked him about the sudden onset of algae on the filter pipe. Is this true? following the local shops advice we started with a group of 5 feeder fish we had for a week before putting more fish in and we stuck with only a couple of hardy fish to start the nitrogen cycle going.
Is there anything I should be doing other than water changed every couple of days?
Also I used bio start when we were starting our tank which I think is basically just the bacteria needed. Should I add some more of that?
Thanks for the help.

Ouch.
Bio start is most likely not going to succeed at anything except getting money from you.
 
Like Berry said, your tank is not cycled, making your fish sick, killing it.
The best way to cycle a tank, is to do a fishless cycle, but since you already have fish, I suggest you do like she said, a fish in cycle.
 
Since you don't have any ways to test your water, I suggest you take water sample to you fish store, and have them test it for you. It will be the best way to see if nitrogen cycle has started.

Please do not add anymore fish!
 
Could you please list all of the fish and their numbers? If there are any fish you're not sure of, post a picture of them and we should be able to ID it for you
 
As the others have said, your tank is, unfortunately, not cycled.
Read these two links to find out what cycling is, they're found HERE and HERE.
 
The best thing right now is for you to get a test kit, I use the API freshwater master test kit which tests for everything you need to know at the moment. The sooner you can get a liquid test kit the better.
Once you have your test kit, you need to do a water change everytime you see ammonia reach between 0.5ppm and 1.0ppm. Ideally, you want to keep the ammonia at around 0.5ppm, any higher and you need to do a water change. At 0.5ppm there is enough ammonia for the bacteria but not too much to cause long-term health issues in your fish.
 
If you can, rehome some/all of your fish, if you can rehome all of them then you can do a fishless cycle (click the cycling link in my signature) and then restock your tank with appropriate fish.
At a guess, your tank is overstocked, so, whether you have all of your fish for your cycle or not, you will need to rehome some eventually, sorry. IMO its better to do it now before you get attached and without making them go through a horrible fish-in cycle.
 
Sorry if this sounds really harsh.

They tested it two days ago and said everything is fine except for a very slightly high ammonium level which wont harm the fish providing we get on to it with water changes every two days for a little while. From my understanding of the nitrogen cycle with this amount of fish and weekly water changes the cycle must have started for it to test fine
The bio start came with the water conditioner and is only a little bottle and the pet shop said its not that critical but helps a bit.

sorry missed your post. No its not harsh I wish I had of known earlier you could do a fishless cycle. I did know somewhat about the cycle but didn't know it took so long.
We had: 14 guppies (some babies), 1 misc feeder fish, a huge bristlenose, 2 tetras, 5 neons. I think we are down to 10 or so guppies. Haven't lost anything else. We also have 5 or so live plants. We have had the tank for probably a month now so with that amount of fish I would expect it to be underway. I think I need to get a nitrate or nitrite test. Already have most of the tests.
Thanks.

Did you get the exact results of ammonia, nitrite, nitrate and pH?
 
Could you please post pictures of the feeder fish and two tetras? If you can't upload them to the forum, upload them to a photohosting website like photobucket.
 
The API freshwater master test kit has all of the test you need to know

Will upload requested pics to this folder. Too big off an slr to upload here.
http/srdv.ms/1bYmsPO
Ammonia was about 0.25, pH was 7.2 and don't have the other two tests but will buy asap.
Thanks for the help. None more have died yet which is good.

Also is a bourbon bottle in the tank a problem? I have removed all stickers and glue, except some white writing which is not coming off even with a wire brush. I also rinsed it multiple times with hot water. I have googled it and haven't found anyone saying its harmful and a good friend of mine who has a salt water setup said its fine. But the lady at the pet shop says it will kill them. Can anyone clear that up for me?
Thanks again.

Sorry, your link isn't working for me
 
I have no idea about the bourban bottle sorry, though it may pay to have it soak in a bucket of bleach (NOT the bucket used for your water changes) and then give it a really good rinse, and then leave it for a few days soaking in a bucket of heavily dechlorinated (3x normal dosage) water. That should clear up any nasties.

Http/sdrv.ms/1bYmsPO
Sorry my computer doesn't like this forum it wont let me copy and paste and also can't find an edit button.

The first picture looks to be of a white cloud mountain minnow:

 
Probably best to rehome yours, as they're schooling fish that prefer colder waters tan most tropical fish.
 
The last two look to be platies:

The minnow seems happy and has been in there the longest without issue. We are still losing guppies but much slower now and I think it might have stopped so thanks a lot for the help I will certainly be checking here before listening to the pet shop again. And will definitely do a fishless cycle for the next tank.