Fellow apartment dweller here. Everyone has different situations but I managed to fit a 4ftx2ft 80 gallon tank into a 500 sq ft apartment and ran RODI line basically around the whole place to make life easier on me. Here is what I did in the past based on a ton of research and trial/error.
1. Get leak detectors. You can find them online and a lot are WifI connected so they will alert your phone and have an audible alarm. 99% of the time you won't need them but a loose RO line or a failed float valve will absolutely happen at some point. If the equipment holds water and is out of sight/mind, always have a way of alerting you if something bad happens.
2. I had the RO Buddy others mentioned here for a bit. It works well but I hated the form factor. I did not want to wall mount at the time so not having a solid base caused more harm than good in my situation since it would not stand up on its own under my sink.
3. The BRS 4 stage system is wide but narrow and IME fits nicely in the very back of even the smallest cabinets under a sink. It also stands vertically on it's own without being mounted to anything which helped in my situation. The cost is elevated compared to other units out there but the unit is solid and will last forever. I think it is worth it in the end and their upgrade packages are nice for when you eventually want to produce even better quality water (if needed)
4. Much like Tuan mentioned above, there are connections you can get to plumb everything under a sink nicely. To manage the wastewater I got a
saddle valve,
this feed adapter for the fresh water in, and bought just the "U" part of the sink drain so I could drill a hole and not worry about my landlord when I move out. What this all means is that I leave the unit fully under the sink and on. I then have a valve where my reservoir is and a float valve. I can turn one valve and let my water fill up with little concern of forgetting it and flooding.
5. Depending on your situation, you can likely drill a 1/4" hole to run the fresh RO line out from under a cabinet and snake the tubing to wherever you need it. I initially just had a line next to my sink to fill buckets, then moved to a setup that ran nearly 20ft horizontally around my apartment, and now have a clean line running through a cabinet, 2 walls, and into a 30-gallon drum in a closet. It was a bit of work but now I never need to carry a bucket and it was 100% worth it.
Of course, this is just my experience and opinion but after 10 years in smaller city apartments and over 5 of those with "large" tanks in those smaller spaces. Happy to lend more advice but I'd say save up a bit and get the right system that fits your space best and can help make life easier in the long run. You likely won't have a tank for less than a few years and the bucket hauling and flooding risk of these sorts of systems adds up over the lifespan of a tank. Do things right the first time so you can focus on more important things.
