I am using a DIY Co2 bottle injected into the intake of my HOT Magnum
filter.It is working
well. My problem is my Ph is 7.2 and my Kh is
4. This gives me a Co2 rate of 6 mlg/liter.
barely more than I would have with no Co2
injection at all.

How do I lower my Ph without lowering my Kh?

Inject more CO2.

I tried to raise my Kh with baking soda but
it raised my Ph,so I gained nothing.

It will - at a given level of CO2, PH & KH will work hand in hand - to be
more exact, PH is a product of KH. As KH goes down, so does PH and vice
versa (assuming constant CO2)

I have been dosing Ph Down but My Ph just
bounces back up.

It will - don't bother with it (!)

My Dh is high as well.But I don't want to get into
RO water and all that expense if I can avoid it.

GH is a very different thing to KH. I am no chemist but as I understand it
Kh is a meaure of the amount of carbonate in the water while GH is a measure
of the amount of mineral ions in the water.

Your KH at 4 degrees is about as low as you want it - unless you want a very
acidic tank. You havn't said what you are trying to keep & what conditions
you are aiming for but if you want a lower PH your options are to either
reduce the KH which you can do by adding some peat to your filter or
alternatively you will have to add more CO2.

The water out my tap has a KH of about 9.5 Deg & my GH is at about 16 deg.
It also has 45-50ppm of Nitrates in it (yeuch!).

I have a rig I built for filtering water through peat - I make about 180
litres at a time & filter the water through a Nitragon nitrate filter when I
fill it up. I set the KH to about 5 degrees and in doing this the GH also
comes down to about 10-11 degrees (which I don't understand why but its no
bad thing so I'll take the deal!)

The resulting water has a base PH of around 7.8 (assuming 2-3ppm CO2.). When
I inject CO2 into it to 15ppm I get PH 7 in my tank.

HTH

I.