water emptying out of bowl

I just installed a new Delta Prelude low flush toilet in a bathroom I remodeled. Now every time I flush the old toilet in a nearby bathroom some water empties out of the bowl in the new toilet. This problem just started after I installed the new toilet. Any suggestions? Thanks.

Common problem, not so easy fix. Older toilets flush a lot of water, but do it slowly. Newer ones flush much faster and the water can skip across to a back-to-back toilet. This can slosh things around, and lower the level. The fitting in the wall worked with the slower flushes, but you need a different one in there to work with any of the newer toilets. The Toto installation and spec sheets warn about this, and I'm sure other companies would have the same problem - they may or may not mention it in their documentation, though.

I was intrigued that the problem is that the OLD toilet is causing problems with his NEW toilet. However, it's pretty clear that the nearby old toilet is connected into the same line as the new one in a way that is causing the issue. Maybe the old toilet that the poster replaced had so much water volume in the bowl between flushes that it wasn't as noticeable or simply was harder to disturb.

Poster: I'm making an assumption that you replaced the toilet that used to be in the bathroom you remodeled with this new Delta, correct? Did you have to change any of the underlying plumbing, or did you just swap one toilet for the other?

I did not have to make any changes to the plumbing. The toilets are not back to back but the drains are only three feet apart.
I just noticed that I am losing some water out of the bowl in the new toilet even without flushing the old toilet. Flushing the old toilet just makes the new toilet empty out more. There is no leak onto the floor in the new toilet.

On new Delta toilet that was installed the the bowl is not quite level. The front is slightly higher than the back. Could this be the reason water is emptying out of the bowl? Flushing the older nearby toilet does make it empty out more though

Toilets are like a teapot...the outlet's height determines how full it can get. If you tilt it towards the outlet, then some water will flow out. Under normal circumstances, the bowl is full, and can only reach a certain level before some leaves out the spout (called a weir in a toilet). So, yes, being off level, tilted towards the back, will affect the bowl water height some, but not usually a lot. If the toilets are not vented properly, water rushing past the connection can cause a pressure wave which can rock the water in the bowl, sloshing some over the weir.