I've had my 150 gl tank now for over two years, fish have come and gone . We've never had much success with Betta's . Our last one sadly diedl yesterday . Colouring still beautiful hard lump in his belly :( when I took him out from the bottom.

I tested the water with new strips I got, (i'd been using the api test kit with drops) and discovered my water KH is really high. We have hard water here. I clean the tank using a python there is no capacity for us to leave water lying around to soften, plus refilling would be an option.

We have mainly tetras, 5 neons, 6 cardinals, 3 black phanton (and some 4 ginger tetras) a few 6 corydorys but my daughter originally wanted an angel fish i want to make sure that the water is right for them. we also have wizard snails, (2) and 3 assain snails - to eat the pest snails) .

What is the ideal KH range. All my other readings are high, my Nitrate is up a tiny bit, usually zero but assume poor betta dying has had something to do with that , so will be doing partial water change today .

We also had an airpump that had two tubes and one has died so I've ordered another one as a couple of the cory dorys I've noticed since the pump failure swim up to the surface (I never knew they went that high) and back down. (new pump arriving from amazon weds a silent a300 which we had before)

I have a 150l tank. My readings today are before cleaning

GH 180
KH between 80 & 120
PH 7.5
Nitrite 0
Nitrate 20
Amonia 0 - ( i always got false positives from tap water with the chemical testing kit strips work for me)

So my questions really cos I'm a waffly cow lol

1. How do I reduce my GH
2. What is the right KH / GH for tetras/betta/angel fish (not obv angel and betta together)
3 . Any reason why my cordy dorys make dashes to the surface?

Any help would really be appreciated.

The only way to reduce gH and kH is to dilute your tap water with reverse osmosis water (available from some LFS, or you can get an RO filter and make your own). There's an article about the use of RO here: https://www.fishkeeping.co.uk/articles_109/ro-freshwater.htm
You'd need to get kH to 60 ish and gH to about 100 for those fish.

I have lots of distilled water xxx could I put that in the tank when adding tap water ?

or should I just keep the conditions as they are x (obv attack the nitrate) x

Yes, you can dilute your tap water with distilled. If you go roughly 60% tap to 40% distilled you'll be about right. Make the change gradually over a number of water changes.

Your nitrate level is fine - anything up to about 40ppm is OK for fish.