Id recommend a used AIO if youre looking to cut costs. A “cheap” tank can be done pretty easily if you have a minimalistic setup without a lot of moving parts. Each thing you add, like a skimmer for instance, costs not only money but time to maintain it. Anyways ill share a few of my “cheap” tanks, and what i bought, and how it all worked out
Heres my old nuvo 40 “budget tank”
Tank stand and return pump bought used for $300
Cheap
amazon heaters and inkbird $80
2 viparspectra lights $280( youd only need one unless you want tons of sps)
2 Powerheads $100
Ato $20 using float valve and cheap container
No skimmer,refugium, media etc
Still $780
A nano is significantly cheaper. Heres a 5.5 gallon nano with a $10 heater, $20 light, and a $10 airpump. Whole system including all corals was about $100
And in the middle my old fluval 13.5 (before the 40)
Used fluval and pump $50
Heater and inkbird $60
Glass lid $10
Water bottle ato $15
Powerhead $50
1 viparspectra light $140
Total ~$335
All of the equipment bought for the 13.5 was bought with me already planning to upgrade. I bought a bigger heater, powerhead, and light than what i needed knowing i was gonna upgrade therefore saving me even more in the long run. Cost to upgrade to the 40 was the tank cost plus a light and another powerhead.
My new tank 75g with sump
Free tank, built my own stand out of wood i had laying around.
$100 used sump
1 extra viparspectra light $140
1 extra powerhead. $75
Return pump $80
Again, no skimmer, refugium, uv, etc
Total cost to upgrade like $400