Your ph is to high, tetras enjoy a ph no higher than 6.5 and nas low as 5.0 .Your water will be a lot more stable there than 7.0 and above.If your kh is to high you wont be able to lower ph easily.kh should be on 3 dkh,dont go any lower or you can run into problems ,regular water changes will lower kH this slowly . And gH should be 0-1 dkh.Cant remember ppm, sorry.By the sounds of it you will have to buy gh,kh test kits.Using the peat in the filter is fine but may not be enough to lower these chems and can be messy.
You can try seting water in a container with the peat,with peat in a pillow case, airate it for one-two weeks or just stir it everyday or so.How much to use is hard to figure. Start out with 2 large handfuls per 30L of wate (dont forget de-chlorinator) and see how it changes ph/gh.Add this water very slowly during partial water changes or they will acid crash.Also you shouldnt use any sodiums found in most ph adjusters,they dont like them.
Cool, think thats it,hope it helps
p.s Your not keeping fish your keeping water,get the water right your fish will be happy.