It sounds like you are wanting to change water manually but have pumps do all the work. I considered such an approach but eventually went with a continuous water change. It costs a little more upfront but I'm glad I made the investment. I also spent a little more money to get an accurate top-off system. Here is what I have:
For the fresh water, I have an RO/DI unit feeding a 50 gallon Rubbermaid commercial trash can. I have a cheap float valve in the trash can to ultimately stop flow. The feed water to the RO/DI is connected to a solenoid valve and the valve connected to a timer. I know approximately how much water my tank consumes and the production rate of my RO/DI throughout the year. Based upon this knowledge, I set the timer sufficient to fill the tank twice per week with a couple hours extra. This minimizes the "TDS" creep mentioned above. I only have to change my DI once every ~3 months so I know my approach is working. I have thought about switching to a high/low float switch system using my Apex, but the current system is working well.
For top-off, I use a Litermeter III with level controller to fill my sump continuously. The Litermeter is set to deliver slightly more water than the tank looses in a single day. The Litermeter delivers the desired amount over many intervals. Because the LMIII is set to delivery slightly more than the tank looses, the shut-off controller activates about once every couple days for a few hours. Once the level goes back down, the process starts again. Even if the LMIII level switch failed, there wouldn't be a problem because the LM is set to deliver only slightly more than the tank looses. It would take several weeks for this event to cause an issue. Plus, I have a second high level switch controlled through my Apex that shuts everything off if the level gets too high.
For water change, I have the RO/DI hooked up to a large 100 gallon Rubbermaid container with a cheap float switch. I have a MaxiJet powerhead in the container for water movement. Once per month, I allow the RO/DI to fill the container. I then mix the salt. Once the salt is ready, two LMIIIs complete the continuous water change. The LMIIIs change ~4 gallons per day of water for my 180 gallon system. The sump contains both high and low level switches that shut-off all my LMIIIs if problems are noticed. I monitor salinity weekly to ensure there isn't a mismatch in pump rate.
Overall, the system above has worked well. The LMIIIIs have to be calibrated occasionally plus they are a little pricey up-front. However, LMIIIs can pump against almost unlimited head or "pull against" ~20' of head so they can be placed almost anywhere. They only use 1/4" tubing so this can be hid almost anywhere.
Another nice thing with this system is the waste water coming from my tank's continuous water change is delivered to a cascading series of quarantine tanks. I never have to worry about having a quarantine tank ready
.
Good luck.