I've actually switched to keeping hard water species in this tank now because despite soft tap water the soil I use considerably raises both pH and hardness. Almost all the commercial composts available contain considerable amounts of limestone, phosphate and sulphate which really push things up.
If this isn't an issue for you then I can recommend Westland's Aquatic Compost, which is what I'm using now. It is sterilised and designed to minimise ammonia leaching. John Innes (mixes 1-3 and cutting compost) should also be fine (JI 3 was what I was using at the start), but expect high ammonia levels for the first few weeks.
If you want to try and keep the water a bit softer, then I'd recommend mixing your own - roughly equal amounts of sphagnum peat moss (ebay), lime-free sand/grit (any garden centre) and fertiliser free topsoil or loam - this last one is difficult to find, personally I'd take a trip to Epping forest and collect some of last winter's beech loam.
You could also try something called 'Original Aqua Soil', if you search ebay for 'aqua soil' you should find it, comes in big blue bags.
I know someone trialling it who say it hasn't affected their water stats much at all, but no idea yet on how effective it is at growing plants!