Thanks for the replies guys it's much appreciated,sorry I haven't posted earlier,but that thing that gets in the way of my life called "work" has been calling!
Test kit is a API freshwater master,but it is I guess about 3 years old now,something I certainly hadn't thought of,but maybe its out of date and giving dodgy readings.
When I emptied the tank I left the gravel there,I did a big water change and had a good hoover round,and I stripped down the powerhead,but nothing really more drastic than I was doing every 2 weeks when it had my other fish in.
Don't think lack of oxygen is a problem as I have a spray bar just out of the water with a really good current moving the surface around.
There wasn't much food to clean up as I only fed them a few times before they passed away.
I certainly does sound like an ammonia problem and maybe the puffers overfeeding on snails caused a massive spike.
I really want to get a puffer,but certainly don't want to loose another one so I think I will get a snail trap and try and get rid of them before trying again.
Any other suggestions on what I could do before adding another one?
Also while I am here what are peoples thoughts/suggestions on other types of puffer,don't mind if they are freshwater/brackish,but don't want a type that gets bigger than 6" as when I upgrade my tank I won't be able to go much bigger than 30/40 gallon.
Thanks again for all the comments
some ideas I hadn't thought of have come up and hopefully maybe a answer to my problem.