once the solutions are mixed does it settle at all? does the powder drop out of suspension?
Nope. It's not in suspension it's in solution. So it cannot come out of solution unless some chemical change forced it to, like if you added calcium into the carbonate or vice versa, it'd immediately start precipitating out, you'd end up with milk, then calcium carbonate powder (coral skeleton dust).
EDIT: Regarding that ^, have your tubing that drips into the sump separated a bit, as even though the dosing pump waits several seconds after dosing one, to dose the other, unless you've got major flow in the sump, the concentrated solutions can still come into contact with each other in the sump. You'll notice it's a problem if you start seeing 'milk' forming in the water.
To mix, I fill my 10L container with RODI, then get a big pan of hot tap water on the stove and sit the container in that, to warm the water. It mixes so much easier into warm water. Then once at about body temperature, I put an airline into it and slowly pour in the powder. Keep mixing, or put the lid on and shake until the solution goes clear, and there's no powder on the bottom, and it's then sorted.
Try to mix the solution as strong as you can, it reduces the workload on the dosing head. For instance, I dissolve 2KG of sodium bicarbonate into the 10L of water, you could probably double that before it became supersaturated and you'd struggle to dissolve any more.
Also if you wouldn't mind sharing your source on the powders you buy? just want to make sure I have the same supplier .
Is there a recomnded length for the dosing pipes, is there any reason I couldn't run pipes at a couple of meters long if I had to?
The shorter the length, the less strain on the head. If you put strain on the head, it can lead to inaccuracy in dosing quantity as some fluid will seep past the rollers under the back-pressure. 2M should be fine but I wouldn't go much longer.
Most importantly, whenever making any change to the length of the tube, you must recalibrate the head. The same goes for replacing with stronger concentrated fluid, as it's denser and will affect the calibration.
how often do you have to detach pipes to clean them? if at all?
There's no real contamination, so nothing really needs cleaning. So long as the tops of the containers are covered to prevent dust, it's just a single chemical flowing through. No bacteria is going to grow, or algae, so there's not really anything to clean.
Whenever I've had to replenish the 10L cans, I've had the head pull some vinegar through the lines, followed by water, but that's just for peace of mind, there was nothing really there to clean.
The ends that drip into the sump can get build up of deposits though, as the trailing drips dry. So make sure you keep those clean otherwise it could block or reduce flow.
I know this question will differ from tank to tank, but approx how much of each item you dose over the course of a week\month?
Not so much any more, as I've been doing huge water changes, so replenishment has far outweighed consumption. At it's peak when I had my SPS, that were growing fast, I noticed consumption increase with every new stony coral I added.
I think at the time I had 1000g of each chemical in the 10L of water, so I'd dose 85ml/day, which is 8.5g/day. Making the solution twice as strong with 2KG into 10L means the pump only operated for 42ml/day. The numbers varied slighly as alkalinity was consumed a bit faster than calcium, and those are from memory, I've got them written down at home...
The process to figure out how much to dose is to measure one week, measure the next, then work out how much the parameter has declined, use the Reef Chemistry Dosing Calculator
to work out how much powder that is needed for your total water volume to raise to the desired figure, then work it out based on the strength of your solution how much fluid that is. Start with that, and monitor through the week, and adjust. Once running, just check the levels once per week, you'll probably have to tweak the dosing quantity a few ml in either direction and it might smooth out, or not if you keep adding corals.