Hi all,

So i am a fairly experienced tropical tank owner. I have several and one of the tanks has just lets say become available because i was winding it down. I am looking to replace the tank with a tropical marine setup.

A - Are existing tropical tanks (120L) easy to convert to marine tropical or would

B - a whole new setup be easier (if i cannot convert i would probably look for a larger tank)?

I ask because i have been trying to read up on what I need/ what is best for the tank, and most setups mention having a sump tank, which i like the look of - is it possible to add to a glass tank a sump tank?

On top of this what do i actually need (a list or recommendations would be nice) because everywhere is saying different or different ways of doing the same thing so what do people prefer?

Accessories wise i have heard a lot as well - everyone recommends a refractometer (US/UK SP?) and whilst i see the value of a handheld one for your incoming water are there not any that could be affixed into the tank system or is this not done in order to protect the light detection surface? i have seen a seneye reef kit and i was wondering if there is something similar to it for saltwater conc?

What would people think about building a system that instead of relies on me draining and adding a load of preheated fresh-saltwater to the tank having a separate smaller tank attached to a feed-in line that would drip add the pre-made up water at a steady rate but it would be at room temperature, then having a kinda overflow pipe that drained the same amount until maybe slightly more than a quarter of the tank has been replaced? (could i even catch and clean this water?)

TLDR:

1. Tropical glass tank became free (120L) should i replace with a bigger one or try to add all the bits?
2. What are "all the bits?"/What would you recommend be "all the bits?"
3. Has anyone seen any decent refractometers that can be built into the tank system? similar to seneye reef kit
4. Are there any better ways of changing the water in a marine tank?

EDIT: PS: i'm in the UK