If its the splashing, then adding an extension to the overflow lip might help. Easy and inexpensive.
Below are some pics for the lip extension for an Aqua Clear 50 ( though it can be applied to most any HOB
). You'll need sturdy scissors, pliers, and the plastic strip found on most glass hinged tops. Most LFS
sell it separately in the lighting supply section. Hopefully the pics are self explanatory. You cut the strip to the width of the HOB
spillway. You want to make two bends (heat helps make the plastic more pliable):
#1. Bend the section of the strip that has the part that grips the glass hood downward about 45 degrees.
#2. Bend the strip lengthwise along the middle about 45 degrees upward.
Next, use pliers to pinch the part that grips the glass hood in one or two areas. Do not apply too much force. This will allow the lip extension to grip the HOB
spillway and not fall into your tank.
You will have to adjust the angle of the bends and see what works best with your water level.