Why is that? As long as you have a cold water supply at adequate pressure you're good to go. They're cheap. A post DI would add about another £20 to the cost. You only need to make less than 100 litres per day. You don't intend to change the water EVERY day do you? I usually change my water every 4 weeks - not every week.
Not true. It's caused by imbalance of nutrients. If you look at MD Fish Tanks on youtube he doesn't use CO2, runs his tanks for 12 hours of light a day and doesn't have that much high flow. He doesn't have staghorn.
I find it really odd that they go to the poor flow, low CO2 excuse for nearly every algae aliment??!!! They want you to up your flow rate to 10X tank volume per hour, some even go as far as put a flow pump into their tanks!!!
If people are so worried about flow they can get a "visual picture" by doing what the fluid dynamicists do: drop a dye to see the mixing. In my case I use calcium carbonate because from time to time my KH drop and I need to raise it again. It's a white powder that takes a few days to dissolve completely, but all you do is to put some in a plastic tub like you get your in vitro plants in and put that near the outlet pipes and you'll see how the water mixes. In the case of my tank with 3X tank volume turnover (not 10X as these idiots recommend), it merely takes a few minutes and the whole aquarium water becomes a uniform white mist. I've got plenty of "dead spots" but somehow over those few minutes the flow found its way to mix everything in. . .
High flow is only a nuisance to plants in the sense that it could dislodge them, and can annoy fish if they're not suited to the strength of the currents.
CO2 itself is a secondary issue. I tell you again that I'm running less than 10 mg/l and I have no BBA, no BGA, no staghorn, etc. . and my plants still pearl. Some scientist states that for natural sunlight at full strength on aquatic plants they only need 40 mg/l CO2, and full strength natural sunlight is a LOT more light than any of our aquarium lights can provide. We're talking about 1000 - 1500 PAR!!!! They only get a 10th of that in our tanks, so why would you want to even put 30 mg/l of CO2, the plants can't even use it at the light levels we're supplying to them.
Like I said, MD Fish Tanks doesn't use CO2 and he grows plants fine.