No sump?

If you run a line directly from your RO unit to the tank, you'll need a float switch ($7-$10) and solenoid valve ($20-$25) (along with home made brackets). 

If you have someplace to hide a 5 gallon bucket, or similar, you can fill it with RO water and use a powerhead ($25 or free if you have one laying around) and float switch ($7-$10) and some home made brackets.

I can help you out in either route if noone has stepped up yet.  Let me know. 

Connie, I think that your project will be easier than you think.  I just added a auto topoff to my new nano tank.  Although I did not have a RO unit, you may find that both setups will be similar except for the RO input.

This is a shot of the whole setup

Wide angle shot of various modifications

Shot of cheap float switch from Internet ($6 I think  http://www.floatswitches.net/floatswitch.html - http://www.floatswitches.net/floatswitch.html )  The bracket is of scrap acrylic and nylon screw/wing nut from Lowes (< $1)

$5 Walmart trash can with 24vdc AquaGlobe pump (?  $15)  Sorry you can't see the pump but here is a link that shows it http://shop.store.yahoo.com/pet-guys/-69991300007660.html - http://shop.store.yahoo.com/pet-guys/-69991300007660.html  

Three week old JBJ Nano-Cube

You would need a reservoir an/or a float switch to control your RO.  I would be happy to assist you should you choose to DIY.  Good luck!

Fred

Fred,

do you ge that much evaporation in your nano cube??  I have one and I don't.  I may add a cup or two of water a week.  Cool idea thou.

Initially, I had a heat problem.  The temp would go between 84f and 76f depending if the 28w PC light was on.  I planned to add another PC so I installed a fan that blows into the tank.  The fan kept the temp stable with both fan and heater working.  I even added a second 28w PC and now the temp is stable at 78f +/- .7 of a degree.  As to evaporation, I was losing about .5 to 1 liter per day.  I was worried about salinity swings due to the large evaporation of water per day.  NOW I am happy with just about ever thing except circulation but I have more plans.....

Fred 

Today, I placed the stock pump (which I had earlier replaced)  in the tank to help out but I do not like too much clutter in the tank and I will most likely move it to the sump.

Fred

No sump... I know.. Its on my 45 & since its not drilled I never bothered. I have a magnum 350 that I use and just clean out once a week or so. It seems to do the trick.  From what I hear I should be ok with horses.... I better be they are SPENDY!!

Thanks guys.... Jon I got a PM from Jeff & he lives closer then you so I am going to have him save me from myself.... Ill let you all know how it turns out...