Old tank syndrome from Diomond Goby?

Be careful with buffering for pH since it always raises/affects alkalinity. 7.8 pH isn't the end of the world, many tanks run at that level at the low pH point of the day very successfully. It's a lot of times worse to dose a buffer than just to let the tank naturally adjust the pH every day. Just keep your alk stable and you should be fine as long as the room is well ventilated. Doing the airstone test is not a bad idea anyway just to see what your CO2 levels are, although keep in mind that's just a snapshot view of the system and could change based on the time of day, if you've had any visitors over, if the furnace is on, etc etc.

Hydrogen sulfide can build up in low oxygen zones in your sandbed so if you had them in the first place and they got disturbed you could have some problems.