I’m sorry if this is a “common sense” question, but I want to make sure that I understand everything correctly before I start up my tank. I’ve noticed that when people talk about doing a quarter or third of a tank water changes they do it from and to the sump.
Is it correct to assume that you turn off whatever equipment you have in the sump (protein skimmer, reactors, etc) prior to the water change? Then turn them back on when it’s done? Or, do I have this completely wrong and you’re supposed to do water changes from the main tank?
During the Water Change you can first remove the old Water and then replace it with new Water. If you wanted to change 30% then the Tank and Sump will now have 70% of the old Water and 30% of the new Water. I do a 10% Water Change weekly and do it as two 5% Water Changes.
Turning off equipment before starting the Water Change prevents issues with Heaters/Thermostats/Pumps etc. not being in Water and running dry and/or breaking and offers User Protection against something going wrong e.g. a Rock falls and the "live" Heater is smashed with you Hands in the Tank/Sump area. Just don't forget to turn it all back on when you are done!
I have a Red Sea Reefer Tank and a Tunze ATO - but other Tank/ATOs will be similar. When the Return Pump is turned off the Water draining in the Weir Box at the top of the Tank empties the additional Water that would normally be replaced by the Return Pump into the Sump. The Water level drops in the Display Tank until Water can no longer flow through the slots in the Weir or until the top of the Return Outlet(s) are no longer fully submerged. It is a good Test to confirm that if the Return Pump was to fail, or there was a Power Cut, that there is sufficient capacity in the Sump to contain the additional Water.
In practice, I perform a Water Change into and then out of the Sump with all the Equipment running. This avoids a change in the Display Tank and only the level in the Sump is raised. For me (and my Fish/Corals) this is less traumatic than have everything switched off - that can be quite noisy and disruptive. This wouldn't work for a 30% Water Change but does work (for me) for a 5% or 10% Water Change. The problem with this method is that some of the new Water is also removed so that the 30:70 or 5:95 or 10:90 ratio isn't quite what it seems - but that is OK (for me).
The System Volume is nominally 625 litres and the Sump is 125 litres. The height of Water in the Sump is approximately 2/3 of the available volume in normal operation. I keep RO/DI Water in 3x 25 litres Containers and have a 30 litre Plaster's Bucket to mix the Salt and RO/DI Water. One of the Containers is used as a Reservoir for the ATO. When it is close to being empty I swap it out for another full Container. Another of the Containers is used to fill the Plaster's Bucket so that the Salt can be mixed with a Pump and later an Aquarium Heater/Thermostat. When I'm sure the Water is mixed and at the Tank Temperature, I pump the mixed Water into the Sump.
The ATO has an Electronic Sensor to switch on the ATO Pump if the level drops - it isn't triggered because the Water Volume has increased in the Sump e.g. I added 25 litres to the System Volume via the Sump. The ATO has a separate Float Sensor that is positioned high enough not to trigger with the additional Water volume. I usually siphon Water from the Sump area into the Plaster's Bucket to remove the same Volume of old Tank Water. I can also siphon Water from the Display Tank if I think the Sand needs attention. The process is repeated for the second 25 litres Water Change.
The ATO has safeguards to prevent too much Water being added to the Sump. In my case the Sump can accommodate either the addition of 25 litres of additional mixed Water during a Water Change or if the ATO should go wrong, the addition of 25 litres of RO/DI Water. As an example, if I start to add the second 25 litres of mixed Water before emptying the first 25 litres, I'm alerted to the Water level rising in the Sump before there is a problem.
I think it is worth thinking through what could possibly go wrong and trying to mitigate it. Knowing that the same volume of Water that is added during a Water Change could also be accommodated in the Sump if the ATO failed is reassuring. A 5% drop in Salinity or small change in Water Temperature could also not be a concern. Testing that the System and or Sump will not overflow in the event of a Return Pump failure is also worthwhile.
After a few Water Changes you get to know what works (and doesn't). I use an Eheim Crook to hook over the Sump or Display Tank Wall to ensure it stay in place. I also have an Eheim Elbow on the Pump so that most of the Mixed Water can be removed from the Plaster's Bucket because the inlet faces downwards. I use a Trolley Dolley to move the Water Containers around as 25 litres of Water is a bit much for me. So whilst Water Changes are "Common Sense" there is still plenty of things to go wrong.
