is live rock nessecary

A key piont--some form of biological filtration is required. Lacking LR, filter media like bio balls, sponges, ceramic noodels, etc are needed in either a wet dry or power filter. You will also have to provide cover for most fish--very few will be comfortable lacking hiding places.
 
i would suggest having some but i have seen some great tanks with just fish in them, but they seem to lack something for me, but you can be a little laxer with the conditions expecialy light in fish only tanks
 
The bad thing doing filtration without LR is nitrates. There currently isnt a reliable fish safe mechanical filter that can reduce nitrates, and most filters are nitrate factories. But you dont need LR to get the same benefits, you can buy large pieces of porus rock like Tufa or Lava rock and it will work just as good as LR, but you wont get all the little critters living on it.
 
Yes, it's a matter of preference. (looks, etc.)
FYI: My 400 gallon is a FO tank.

As said, you do need very efficient biological filtration. And, decor is also needed for the fish. I prefer coral skeletons (not fake) since they also keep the Ph in it's ideal range (I never need any buffer).

Mine is a large w/d filter which keeps ammonia and nitrites at 0 all of the time.
My nitrate filter keeps nitrates near 0 (with heavy feedings, heavy bioload, and no water changes in 6 months).
 
My 55 that is a temporary home for my baby Picasso Trigger has no live rock but it is full of lace, moon, and reef rock which provides plenty of surface area for the good bacteria and lots of caves for Rembrant to play in. I originally added some sand from one of my other tanks to get they cycle started and it took a little longer to cycle than my tank with live rock. I also keep a close eye on all my levels and keep up with regular water changes. I haven't had any problems and love my fish only tank.
 
I will probably be made fun of but I use a emperor 400 power duel bio-wheel filter and I love it 9 )
I got it used ( free lol ) and the wheels were already scuzzed up with bio gunk so I had about 4-5 days too cycle and I was ready too go. I have never seen the tank above .25 the whole time I've had it. I would recommend 1 too anybody that doesn't want too spend a whole paycheck on rock but that's just my opinion... I've had a few guys tell me they went out and got one new and the guy at their local store let them switch out one of his wheels with one of the new ones so they were also ready to go in a week or so. On the G.A.R.F. website they got some stuff too make your own live rock and it's alot cheaper. You just make it up, drop it in the tank and sprinkle some garf gunk on it and it'll start going like any other live rock...
 
Yes it's possible to build tanks without live rock, just like in the good old days when everything died after a few months. The majority of fish only tanks I see are really FOWLR - Fish Only with live rock - they simply work better.
Live rock was considered a miracle when it became popularised as it made everything so much easier, and I'm not sure I see the point of reinventing a (broken) wheel
 
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