Wow! That escalated fast mate
No experience on the regulator I'm afraid but I will be interested to hear how you get on with it, I also have it on my watch list on Ebay so wondered of they fair as a lot of people who fancy taking the plunge into co2 are frightened off by the initial outlay so if these are ok then its a cost effective entry into it so I'll be watching this thread. I think you can also get these from Ireland.
As for co2 diffusers the worlds your oyster. Lot of people using the
Neo from Aquarium Gardens and recommend it although never had one myself, I will be getting one soon for a nano cube. Quite a lot of people also use inline diffusers but lately I've been hearing mixed reports about them cracking and leaking. Ultimately it breaks down like this, the smaller the bubbles they create the more surface area they have and diffuse better in the water. You tend to find that to get tiny mist like bubbles you need to force the co2 through fine pored ceramics at high pressure. This has the downside of working at high pressures so first thing to find out is if your regulator can work at these pressures and secondly all your connections need to be sound as any small leak will stop it from working. These include the Bazooka style and inlines.
Diffusers that don't need to be high pressure tend to generate bigger bubbles and are a bit more wasteful of gas, dosing co2 is already a wasteful experience as most of it just leaves the water without doing anything. You can buy the cheapish glass ones off ebay usually from China if you want to wait for them coming. I've bought many of these including higher priced UK shop variations and TBH I've never noticed any difference between them, they all seem to generate the same sort of sized bubbles. Only bonus probably with getting cheap Chinese ones is you can get maybe three of them so you can have one cleaning in bleach while one is running and a spare as you will definitely smash one at some point.
That brings us back to the Neo which is acrylic so not as breakable, I've also used the Tropica one which is also acrylic and has a built in bubble counter and check valve so less connections=less chance of leaks and you can buy spare ceramic discs to change out. I had one running on a 100ltr tank to good effect so should be fine in your 70ltr. As for tubing from what I hear it needs to be co2 proof, not sure how much co2 is lost through normal airline of if it's worth worrying about but the thing with co2 is it tends to make plastic go hard and brittle. If you are going down the road of glass diffusers soft co2 tubing works out best as it goes on easier, forcing tubing on to these things is what breaks them shortlt followed by dropping in sink
2kg should last you a long time in your size tank, obviously it all depends on how much you dose but in my 100ltr I would get about 6months from one and that was heavyish dosing. In your tank if you keep the existing lighting there shouldn't really be high demand for co2. Lighting and plant mass drives demand. You may also need to look into your fertiliser regime again when dosing co2, co2 speeds up growth and puts extra demand on nutrients with it which is why a lot earlier on in the post I was suggesting an all in one fertiliser mate. You need to have some of everything in there so adding them all separately and trying to work out if something is missing will turn into a headache and problems will show up quicker with faster growth and heavier plant mass.
Dry salts are the future mate and most cost effective, then you can forget about dosing and concentrate on getting the co2, cleanliness and distribution right.
