Linking Tanks

[seraph]

Hey guys, I have a 120 gallon and 2 75 gallon tanks sitting next to each other.
The idea is to link the water supply together to maximize the water amount, so the 75 gallon tanks will have essentially 270 gallons...
The problem is...the 75 gallons are a good 6" shorter than the 120 gallon...I don't want to link up the water and drain the 120 into the 75s and over flow them onto the floor. I assume I would need to use overflows but i'm just not understanding how they work. What is the best way of linking them?
My filtration is 3 canister filters (and aquaclears) that i'm thinking of putting the intake in one tank and the output in the other tank to daisy chain them. but again the case of overflowing...the 120 does have 2 drilled holes in the back that are currently plugged if that makes it easier to link them up...

Thanks guys

 

[DrawnToBeaches]

tbh this would be extremely difficult without them on a rack system, i have setup a fully connected rack before all with bean animal overflows and a very large sump, what you are proposing is next to impossible without expecting a spill, you would even benefit from racking the 2 75s and having the 120 as a display next to the rack with a large[55-90g] sump next to it, it would also need one heck of a return system, the benfit of this is only using 2 pumps and one center filtration, you can get yourself 2 1k+ gph high head pumps for about 150 each, hook up one return to the 75 that is on the top rack, have that one drilled with on overflow that overflows into the bottom 75 and then another overflow that leads back to the sump with filter socks, then for the120 you'll have a pump dedicated to just that, I recommend using bean animal overflows, they are siphon breakers and are EXTREMELY silent, not to mention the flow is very high and with ball valves you can fine tune it to your pump power. this is similar to what i did for my brother but he had me hook up 4 60g to one sump, racked them one above another and used two pumps, it works really well is silent and so customizable, if you're working with displays however it will be much harder

 

[DrawnToBeaches]

forgot to add if you wish to use the overflow from the top 75 to give flow to the bottom 75 you should either put elbows on the inflow of the bottom 75 or drill a hole in the back or side and bulkhead it and attach the outlet of your choice, may seem confusing but it really isnt once you start planning

 

[Owain4]

I saw a guy on here that built a air tight and water tight tunnel from one tank to the next. it was built so that both ends would be in the water and the main tunnel was above between both tanks. his fish would swim through the tunnel into the different tank.