Arghhhhh nightmare.
You have gotten away with murder so far not your fault fish shop seems to have given you some horrendous advice.
Basics for a marine tank as follows:
Tank obviously
Live rock equal to at least 1 kilo per 2 imperial gallons for your tank you are looking at about 50 kilos of live rock.
Lighting at 2 foot deep for fish only you will be fine with flourescent T8's but if you want any easy corals you will need a bank of about 4 T5's or you may wish to consider TMC's new aqua beam lighting range based on LED's these apparently sit between T5's and metal halides in terms of out put but are very efficient to run.
RO unit (
www.ro-man.com) or a supply of RO water
Good quality synthetic salt for mixing replacement water for water changes plus 2 identical large buckets one is for mixing the new water the other for removing exactly the same amount as you add this prevent changes in salinity over time. Also get a sump pump to put in the bucket for mixing new salt water this can then be used to pump the new water in to the tank after the waste water has been removed.
A refractometer for accurately measuring water salinity these are not sensitive to temperature change hydrometers are useless.
Pumps to turn over the total tank volume at least 20 times per hour if not more you want stream pumps like the tunze there are cheaper available.
I'm trying not to give you overload with this above are the basics without even getting into the chemistry.
Points that concern me are the use of artificial filtration as well as the use of an under gravel filter on a marine tank this will clog over time and cause major problems.
I am also concerned about the fact you have a tang in there at the moment these fish are very susceptable to marine whitespot therefore I would suggest you get the largest UV sterilizer you can.
You also need a big protein skimmer.
Products to consider
TMC V2Skim
TMC V2UV sterilizer
SEIO Hydor Tunze circulation pumps
Wave equipment as sold by
www.stm-shop.co.uk
Also go on the
www.stm-shop.co.uk website and have a browse through the sort of equipment listed if you are anywhere near Sevenoaks I would seriously recommend you go on a shopping trip there with the complete breakdown of what you have got tank equipment wise. Explain what this other LFS has told you and ask for help. This shop is fantastic it is run by hobbyist for hobbyists.
Also subscribe to a fishkeeping magazine and go through the marine set ups that are virtually always featured and look at the tank breakdowns that will give you an indicator of what the current top spec tank are run on and I am afraid canisters and under gravel filters are a very rare sight if ever!