Just keep an eye on it! You are on the high end of it! Ideal is something like 8.0-8.4. If you need to lower it you could try adding one mL of distilled white vinegar per gallon of tank water which will help you to reduce the pH by about 0.3 units! Maybe someone else can shine in on this!
Unless you have added a buffer or kalkwasser to cause it to elevate, then I would suspect testing error. 8.4 is not something that will just happen on its own without help. If you using a pH probe and hasn't been recalibrated in the last 30 days that would be the first place to start. As they age and get dirty they will read high up to .2 higher than actual.
If you are using Kalkwasser and your pH hase been recently calibrated 8.44 is fine. Things don't go south until over 8.6.