I can think of 3 typical reasons a res would be getting a buildup of "something"

1. it's an ongoing issue like if the res wasn't sterilized from a previous grow it might be some sorta mutated strain of fungus or bacteria that's somewhat resistant to your normal methods of upkeep like h2o2...rare but possible just bleach (any sterilizing agent will work but bleach is good cause in small doses it's not bad for the plants and it's cheap) the res between grows

2. Could be temp but at 67 yer probably fine I noticed immediate buildup when the res was over 72 I don't really think the res should even get into the 70's personally but it depends if the root chamber is seperate from the res chamber...if you have just a res that the roots hang down toward the water if it's under 70 you should be fine but if you have a res and a seperate root chamber you might want your water closer to 65 or even lower if the room temps are high because the root chamber will raise the water temp and spark growth...knowing what type of system you have would help...is your res just for each pot/plant or is there one res that pumps to several stations? your res might stay relatively cool because it's a large body of water but dark containers might absorb heat fast enough to get relatively warm

3. light...this is pretty simple to resolve stop all light from entering your res and the root chamber as well (if you have one)

If you have a picture of your system or you could describe it a bit better along with some of the important factors like room/air temp, ventilation etc