Series of water containers in the garden

Hiya folks,
Interested in people's experience implementing above-ground water containers in a garden and in particular the plumbing parts I need to find.
To explain: I have a garden with some slopes and different levels (terraces) and the roof has a good gutter system but the downpipes currently go below ground. I also have compost toilets so all the kitchen and bathroom water that currently goes into a sub-standard septic tank could be diverted and the septic tank removed/filled.
My idea is to install at least 8 above ground water containers (400l to 1,000l are sizes of galvanized/plastic containers it's relatively easy to source as I get money) as I can afford them and divert both my house waste water and gutter water into them - setting them up using poubellarium principles (plants for cleaning and fish for mosquito control). I love the idea of dipping tanks to be able to water different spots int he garden when it gets really hot and the extra habitat/environment created.
I really want the tanks to be connected, allowing to gravity feed and top up the tanks as I use water in the house or it rains.
My question lies in what to use to connect the tanks when there are maybe 10 or 20 metres between them - what kind of seals/connectors and what kind of tubes. I look at the plumbing stuff in the DIY shops and feel pretty lost tbh. I don't like the idea of kits because they are for very specific applications plus all plumbing stuff now costs a fortune so it feels like a waste of money (I'm on a very tight budget).
Have you done a series of connected containers and how have you set it up? I'm in Europe/France not USA - I think products here are very different.
Thanks very much!!