Thanks, just checking it wasn’t a Bead.
Put a T in between the Carbon and the RO intake and put a valve in the teed of piece to control the supply water going in. Other feed in is the RO output.
Looks like you have already taken the KH down a fair amount already.
You will need to do a bit of playing to get the mix of supply and RO right.
One approach is to simply continue to feed in RO until the pond KH gets down to around 50ppm and then start mixing but personally I would suggest that you start with a mix that gives you a combined KH of 40ppm for the water going in, to give you a good chance to see how the system performs, this approach takes a little longer but the changes in the water are more gradual..
When you get down to a pond level of KH 40ppm you will need to keep a close eye on things, it should stabilise at something less than 40ppm, what this number will be depends on the particular system, the filters, stock and the feeding and the amount of mixed water you are feeding in.
If the 40ppm mix is too lean/the amount you are putting in is insufficient your KH will end up at zero and you will crash the pH so I’d be looking to tweak the mix up to 50ppm if the KH in the pond fell below 30ppm, if this fails to address the decline in KH then you will have to go for a stronger mix, Aim is to find a balance where the mix/amount going in is sufficient to compensate for the KH burn of the system and you will have a nice stable KH.
Your KH kit needs to be your best friend throughout all of this, and it does require a fairly disciplined approach to testing, good idea to keep a diary in which you can jot down the numbers etc so you can review how things have progressed.
One thing to remember, if you increase the feeding you will probably need to tweak the mix!
Hope this helps