plumbing help mistake

The other day I added a 20 h to my 75 gallons 40 gallon sump. I drilled 3 holes at the top. I have 1 return and 2 drains. The second drain is only there for a backup since I valve my drains down which keep them very quiet.The Second Drain Stays All The Way Open and sits a bit higher so if I close the valve to much the backup will drain the rest. On this second tank I used the same method. It's a small tank so I went 3/4. Now the mistake I made was out of the bulkhead I put my valves then then I used a t to connect the 2 drains since I'm not worried about any issues in the plumbing. I figured the drain would almost act as a vent so it should flow easier like your drain at home does. But it back fired so bad it makes the drains so loud the gurgle get full of air. I'm using a mag 9.5 which runs 2 returns on my 75 and the 1 on the 20 which is open only part way. This is glued in so if I have to redo it I have to do the whole thing. It would probably have worked better if I put the t in further down the line. The drain keeps up no problem and is almost half closed so the water level doesn't make it gurgle.