I looked all over the internet for ATO systems and DIY top-off projects and everything... I tried peristaltic pumps and spent a lot of money before someone finally convinced me to get the
Tunze Osmolator system. I know that spending around $200 for the Osmolator sounds ridiculous, but it's one of the best investments I've made in my tank. The optical sensor is incredibly reliable and sensitive to TINY changes in water level. My sump is 30" by 12", and I can take out about 12oz of water and then the Tunze kicks in the pump and tops off the water I took out. I can't even see a water level change in the sump taking out so little water, but the Osmolator senses it and replaces the water. It's incredible how consistent my sump level and specific gravity have become. Also, the pumps cost $20 to replace. I don't run my Osmolator pump in kalk or anything like that, so I trust mine will last a long time; still, it's nice to know my next pump will only cost $20 if I ever need to buy it.
Also, the Osmolator has a float switch that is mounted upside down. This is used to sound an alarm if the water level in the sump should get too high. If my return pump should ever fail for some reason, my sump level would rise and the alarm would let me know that the pump had stopped.
I use my Osmolator through a
kalk reactor, which guarantees that I will never overdose kalk unless there is some sort of leak in my tank or other reason why lots of water gets out of the system. So, it makes kalk dosing really easy.
I could keep going about all the reasons I love my Osmolator, but my point with this post is that I spent a lot of money and time trying to get an ATO for really cheap. I ended up with water on the floor a couple times, and I wish I had just bought the Osmolator the first time. It really makes life easy for me. Is it 100% guaranteed to be trouble-free? No, of course not. But I feel it's much more reliable and makes a tank much more stable than any other system on the market. It's not cheap, but I think it's the best ATO product out there on the market.
If you do want to try a DIY top-off system, make sure it has redundancy. Float valves can stick open, etc... So make sure you understand what potential problems are, and then work to reduce the chances of them happening as much as possible.
Sorry for my thesis-long post, but I've had bad experiences with "cheap solutions" and DIY projects. Nothing in this hobby ever seems to be cheap, and I think that trying to take the cheap approach or DIY to save some money with something as important as water replenishment in a reef ecosystem is a calculated risk that could be avoided by buying an ATO system from a company known for quality and reliability.
-Joe