It depends on which equipment.
Here are what I think is good for buying used:
Used tanks that is less than 5 years old are pretty good deals, as long as they do not leak. For saltwater, the glass is going to be scratched sooner or later.
Reef Octopus skimmers, they should be good for a while, as log as the pump is good, the body normally last for a very long time.
Sumps, as long as it holds water.
APEX, they last a good long time.
Eheim return pumps, they last.
Jebao and Coralbox DC return pumps. They are very well built and many higher end DC pumps are actually rebadged Jebao.
ATI t5 fixtures, the parts are cheap and easy to repair.
Here are something I will never buy used:
1. Any Radeon led lights. They over drive the led chips (which also make it less efficient. See Sanjay’s review on G3 pro), that cause led chips aging a lot faster.
2. Kessil led, chips can burnt out due to ineffective heat removal. Also you cant really tell if some chips are burnt out due to they are densely packed.
3. Anything made by Hydor, poor quality especially their pumps.
4. Used Jebao RW, PP series wavemakers, they have thermo chips built in to the motor and they will fail sooner or later.
5. Older Maxspect Gyre pumps. The 100 and 200 series. Their magnet can rust, power cord exposed right next to the motor, propeller fins shatter, rubber gaskets failed with the shaft meets the propeller. Failed controllers, noise.
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