I think Bryan hit it right on the head... If the owners only want it to run and look good, but not participate in a minimal maintenance way... you are not going anywhere fast. If you make it easy for them to do water changes with ro water on a frequent basis.
(Ok, lets just simplify this... everyone here is going to tell you that ONLY RO water should ever be added... in top off or water changes... ever.)
If the are asking you to visit often, (daily) you need to weigh that against your time, distance and cost...get enough to cover your time, since you will be someone else's "tank bitc#" - And we may read about you in 6 months when your burned out, they don't care about you or want to help with the tank...
Lay down some rules... or guidelines. You will need to TEACH them. Not all at once...
The goal is to transfer responsibility to them over time. Teach a man (or woman) to fish... you know... they should be around when you do the work- to learn and see what you do. If they're no-where to be seen and at the club playing golf.... my price would double. They should learn how hard it is to fight to get it back in order, then they will appreciate the smaller amount of time later instead of letting it go to heck again.
Make sure you sit down and do the math for/with them. If they don't see the numbers... RO supplies... membranes, cartridges, water costs, salt to be consumed with water changes, phosphate sponge, and other consumables and their values... your kidding yourself. This isn't a cheap hobby... and while you may think the last person was not doing their job because they were a "dumb reject" or something... maybe they could not get the funding or support for what they really needed... Sure, water changes are the immediate answer and adding a RO, and a skimmer... but I did not see one person look at that this may be caused by lights beyond their useful life, or overfeeding on the owners part leading to high nutrients...etc, etc, etc... You may need to take a step back, re-analyze the situation, and decide... what's best for them, and the livestock... and can they do what they want, with what they have, correctly, and as efficiently as they want to. You may come to a brutal recommendation that they should go to freshwater... or remove the tank altogether. (or maybe they need a mushroom tank, that is nutrient rich, with actinic light and 3 small fish and a shrimp in the tank... I don't have this answer for you..
We need alot more information.... alot more... from where I stand...I hear someone telling me that they want to climb everest, I don't know or see your expertise, nor do I think you realize how high and far you have to go.
Parting thought to put this in perspective... I did this. I spent nights pulling algae until 1am... alot of us have... to go from this... (and this is after 4 plus hours of cleaning...)
<a href="http://s28.photobucket.com/albums/c205/jmkarcz/Tank%20Photos/?action=view¤t=IMG_1933a.jpg" target="_blank"><img src="http://i28.photobucket.com/albums/c205/jmkarcz/Tank%20Photos/IMG_1933a.jpg" border="0" alt="Photobucket"></a>
to this...
<a href="http://s28.photobucket.com/albums/c205/jmkarcz/Tank%20Photos/?action=view¤t=IMG_9524.jpg" target="_blank"><img src="http://i28.photobucket.com/albums/c205/jmkarcz/Tank%20Photos/IMG_9524.jpg" border="0" alt="Photobucket"></a>
I know you don't have coral... but it's still a battle.
Just a thought... or six
.....if your gonna get paid.... and we're going to be your sounding board.... and give you advise.... I think Bri-Guy is raising funds for a new PAR meter..... to benefit everyone... including those seeking help...Like yourself... you cant buy a subscription to a magazine, for what he's asking...