Sounds like you're making the common beginner's mistake of attempting
better aquaria through chemistry. Belay using water additives until
you have a good understanding of your own water and water chemistry in
general. The only thing you should add to your water is something to
remove chlorine/chloramine, a need which I'm guessing the "Kingfisher
British prof safe guard" will fulfill. For years I've been using a
product called Genesis from Aquarium Pharmaceuticals, which does
nothing apart from removing chlorine/chloramine and requires only a
drop/gallon, which makes it very cost effective.

To reiterate, keep it simple and work with the water you've got. My
guess is that your problem isn't pH at all. If it drops below 6.0,
then worry about it; otherwise, I'd say your problem is that your tank
hasn't cycled. I would strongly recommend you check out the
rec.aquaria FAQ at http://faq.thekrib.com/ and learn about the nitrogen
cycle. It's the single most important thing you can learn about
aquaria (and the biggest killer of fish). My guess (which will make
sense after you read the FAQ) is that ammonia is killing your fishies.

Another tip: any aquarium shop that's telling you that your "salt is
high" for a freshwater aquarium is doing you more harm than good if you
listen to their advice. Don't assume they know what they're talking
about; I've worked in such places and know better. Go buy yourself
four test kits: pH, KH, ammonia, and nitrite. You can likely find
yourself a test kit that encompasses all of these. Understand the
nitrogen cycle (ammonia - nitrite - nitrate) and the relationship
between pH and KH. That's 99% of what you need to know to be a
successful fishkeeper. If this is too much work, then please the fish
at the store -- otherwise you're treating your fishes to a slow death.

Be well,
BRiaN