It can range from pretty easy to very complicated depending on a number of factors.
tank design/cabinet/equipment
what type of marine tank you want - fish only or reef
A basic fish only setup will require little change (and I would recommend this before attempting a reef tank).
You will probably need to do the following
- change light tubes for marine
- buy a suitable amount of live rock for filtration (not cheap) or a large external filter to act as a biological and mechanical filter.
- thoroughly clean the tank and any equipment you are re-using (e.g. heater)
- either remove your current filter, or buy new filter material for it - possibly just for use as a chemical filter.
- buy some crushed coral/coral sand to use as substrate
- buy salt and ideally reverse osmosis water
- add some powerheads for water movement
- buy a protein skimmer (this may require you to DIY your tank lid
- a basic marine test kit and hydrometer
A reef, or more complicated system may also need
- increased lighting
- sump/refugium
- more powerheads
- a more expensive test kit
- other external equipment (e.g. UV steriliser, calcium reactor, de-nitrator, chiller etc.)
It is also important to check that your tank is salt water friendly, with nothing that will corrode or leach into the water.
If you provide more info. on what you would like to achieve, we can help more.
and you will have to start from scratch in terms of filter bacteria as fresh and salt water ones do not transfer (although if you use live rock, little cycling may be necessary).
