First Problem With New Tank

evolution aquarium pure works wonders in settling
tank in well worth a punt tenner a tub

it will help sort the crisis out it contain enzymes
and friendly bacteria suspended in a special jell
works wonders i would get some tomorrow put about
ten pieces in test 24 hours later you will get a surprise   

it clears cloudy tanks

There is a school of thought that says water companies have got better at killing bacteria in recent decades. Since the source of the filter bacteria is the water supply, maybe it's that fewer of them are making it through to our tanks.
Then there are the fish. There are more species available now, and most of the newer ones are more delicate. And the ones that have been available for decades are often now bred in fish farms where they can be dosed heavily with antibiotics, then when they are put into tanks with no antibiotics they fall prey to every infection going. And some breeders use any fish for breeding from regardless of the health of the fish, resulting in weaker fish appearing in the shops.

My fish have been pretty resilient getting through this cannot complain about them but they may be complaining about me!

Reading further about water quality I note that I also have very soft water and therefore may be lacking carbonates. Should I add crushed coral to see if pH stabilises and possibly tank cycles quicker?
Thanks
PS ammonia has stayed below 0.25 for first time overnight and water clarity improved again.

The filter bacteria do need carbonate in the water to multiply. If you have very soft water (ie low GH) it is likely you'll also have low KH. You could take a sample of tapwater to a shop and ask them to test the KH.

If you do have low KH, crushed shells, as biffster says, will increase KH, GH and pH slightly. As will anything made of calcium carbonate such as limestone and coral. If there's room for it, a bag of crushed coral in the filter would do the job.

The other thing you could think about is using remineralisation salts of the kind added to RO water. This would necessitate using the salts at every water change though as swings in GH in particular are bad for fish.

I have KH 3, and since he lives only a few miles from me I expect biffster also has low KH. This was only a problem several years ago when I got very lazy and didn't do water changes for two or three weeks - and suffered a pH crash. Doing water changes every week without fail has kept things stable since then.

Sorry it is a 1501 CristalProfi filter