Just Having Such Bad Luck :(

[Xraymark]

Most bases seem covered.
Only thing maybe how they were acclimated, how long etc. Maybe a big ph difference between ph from LFS and your tank. I test the ph in the bag which gives me an idea if needs a longer process.

 

[Ny82]

I have the filters as high as possible so theres a bigger drop for the water that comes out the filter so on impact it makes bubbles.
Can I ask a really stupid question-please nobody laugh-or if you do don't tell me!
What's a powerhead? Is it this thing?

http/www.ebay.co.uk/itm/AQUARIUM-Marine-FISH-TANK-3000L-H-POWER-HEAD-WAVE-MAKER-/380367426464?pt=UK_Pet_Supplies_Fish&hash=item588faa53a0#ht_2475wt_936

Is this all I need?

 

[snazy]

Even though the waters white-I have just noticed one of my corys acting strange. He is zooming around the tank at some speed. He is actually going flying around

I would not buy fish for a while until you are sure it is not something to do with your tank water, oxygen problem, or sickness of any kind.

You did a 70% water change that morning? What dechlorinator are you using and at what dosage?
If you also are having oxygen problems in the tank, overdosing with certain(most) dechlorinators will additionally deplete the water from oxygen and cause "unidentified" fish deaths.
Corys will probably be the last to die from this as they have a labyrinth organ and will breath air.

Also, fish at the surface, or hiding at any part of the tank may indicate they have a sickness that haven't manifested it's external signs yet and moving tanks will just kill the fish, so watch out your tank for a while just in case.

 

[guppi]

I have the filters as high as possible so theres a bigger drop for the water that comes out the filter so on impact it makes bubbles.
Can I ask a really stupid question-please nobody laugh-or if you do don't tell me!
What's a powerhead? Is it this thing?

http/www.ebay.co.uk/itm/AQUARIUM-Marine-FISH-TANK-3000L-H-POWER-HEAD-WAVE-MAKER-/380367426464?pt=UK_Pet_Supplies_Fish&hash=item588faa53a0#ht_2475wt_936

Is this all I need?

Yes that's a correct.

 

[Ny82]

Even though the waters white-I have just noticed one of my corys acting strange. He is zooming around the tank at some speed. He is actually going flying around

I would not buy fish for a while until you are sure it is not something to do with your tank water, oxygen problem, or sickness of any kind.

You did a 70% water change that morning? What dechlorinator are you using and at what dosage?
If you also are having oxygen problems in the tank, overdosing with certain(most) dechlorinators will additionally deplete the water from oxygen and cause "unidentified" fish deaths.
Corys will probably be the last to die from this as they have a labyrinth organ and will breath air.

Also, fish at the surface, or hiding at any part of the tank may indicate they have a sickness that haven't manifested it's external signs yet and moving tanks will just kill the fish, so watch out your tank for a while just in case.

I use the interpet bio active tapsafe. One inner capful does 45 litres. But as I only did 70% and my tank is about 64 litres, I would say that's all I emptied so only put one inner capful of that.
The tetras, danios or corys have never been up the top circling.
I have 2 filters with those lines breaking the water I don't know how it could be lack of oxygen. The stream of bubbles from each filter goes about 2 or 3 inches under the water and theres 4 streams on the big filter and 8 on the small one.
I don't have any medication apart from gill flukes and swimbladder. Should I get some?
I have ordered that powerhead and also the fluval u2 filter.
Thanks everyone for the replies, talking about it really makes me feel better