The only stupid question is the one not asked when you don't know and want to. We were all beginners at one time.
Carto tanks were a natural evolution from the first tank = MAP tank. The original CE2 cartos were a fillerless tank/carto all in one with very small capacity. Most Angry Pirate (MAP) was doing lots of driving and figured out that he could remove the guts from a CE2 and put them in a larger tube with endcaps that he fabricated. He then began to sell these (called MAP Tanks). He also realized he could do the same thing with a plain carto and just punch some holes in the sides of the carto to allow the juice (held in the larger exterior tank tube) to feed into the carto (so you don't have to constantly refill the carto). Those tubes held about 3 ml of juice plus the juice originally filled in the carto for first fill. Someone else came out with that plain carto tank idea before MAP could market it and the idea just took off. Now there are many different versions.
The "standard" (as often referred to) are designed for regular sized cartos (510 and the 801/fusions). The DC tanks (Dual Coil carto tanks) are longer and often a bit thinner and made for the longer versions of the dual coil cartos. The longer XL length cartos can also be used in the std size tanks but some of the carto length will stick out the bottom and you have to make sure you punch the holes in the carto within the internal space in the tank (where the holes will sit between the top and bottom caps).
There is a little bit of a learning curve with the various tanks. Obviously, some tanks are better than others. As with all things vaping related, various tanks and the various cartos you can use in them, are all very subjective. you can look here to see some various versions of fancier tanks:
Tanks « Vapeporn the most basic ones usually just consist of delrin bottom and top caps (with a hole in each to pass thru the carto) and the clear poly or acrylic exterior tubing (all to hold the juice supply).
The Riva tank systems, I think - do not have them - are made for the Riva and I think they are very different from what we are discussing here - other than they are some sort of tank system to hold a bit more juice and reduce the frequency of refills needed.
There are many different tanks and you can just look at some of the different venders to get an idea of some of the different models. There are also many threads on tanks. If you go to the search link at the top of the page and choose "advanced search", then enter the word tanks and choose thread titles, you will see many threads you can read. Some venders are now stocking some chinese knock-off versions of the basic tank models so those are super cheap (~ $12-$20) and fancier tanks can be priced as high as $70 for ones that are all pyrex glass (even the end caps) and made on a glass lathe. There are even some complex tanks (usually metal with complex designs) that can run over $100. Of course there are versions with prices in many ranges between the cheap and the pricey. They all do pretty much the same thing but some just do it differently in terms of how you refill them, replacing topcaps or refilling without removing the top caps and just using needles or syringes or a new filling assist gismo called slap yo mama (great little invention). Some are prettier and others more plain. There are also now some tanks being designed to be used with atomizers instead of cartos.
I cannot educate you completely about tanks in one response but have a look around at various venders, search ECF and you will get a handle on what these are and the many variations. Some are all glass, others have stainless steel endcaps, some are made of delrin and others have poly-carbonate or acrylic tubes for the tanks. Some juices can crack the tubing of the poly carb or acrylic (and there is a thread dedicated to listing those juices) so some tank makers went to glass.
As for Dual Coils on your Riva, you CAN use them but I suspect you will find them disappointing. Dual coil cartos are usually LR rated and have two much higher ohms coils wired together in parallel. This results in a Low Resistance read on a multimeter or on PVs that have built in resistance readings. The total power being sent to the carto is split between the two coils resulting in a cooler vape. Because the parallel wiring reads as LR, they demand higher amps drain on the batteries and I doubt your Riva has the maximum Amps push capacity to fully power the DC cartos. Actually, because they are two higher ohms (for example, a 1.5 ohms rated DC carto is actually two 3 ohms coils wired together in parallel) they can perform quite nicely at much higher voltages.
Like EVERYTHING vaping related (means ALL vaping gear, ALL juices, etc.) ALL is 100% subjective and a matter of personal preference. Many people love DC cartos. Personally, I think they lack flavor and I prefer single coil ones. In my case, I like bacco and bacco menthol juices ONLY in attys and I have a preferred atty just for those juices. I prefer my fruity and sweet juices in cartos and I have my preferred carto version as well. For the most part, I now use carto tanks over 95% of the time and have several of them running on different devices and each has a different flavor juice in it. Other people will never use a carto or tank because they are dedicated to dripping in atomizers. Unfortunately, no one can tell YOU what YOU will like best and so you just sort of have to keep trying various combinations to see what YOU like best for YOUR juices. every atty and carto model performs differently and you have to try many to find the ones YOU like best of those too. Do not be surprised if your preferences change over time. For my first year of vaping, I was using only tobacco and tobacco menthol flavored juices and I was ONLY dripping in attys (my preferred Joye 302 version - I had tried pretty much every att on the market back then and only really liked this one model). Then I got into DIY (Do It Yourself) juice mixing and changed away from baccos and menthols and now I vape mostly fruity juices and use mostly cartos in carto tanks. As I indicated above, I have tried many many different carto models and found the one I like best.
Hope this is somehow helpful and good luck as you start expanding your vape experience horizons.
