Just looks like a very old tank to me...def. showing all the signs of being 30+ yrs old. Even looks to have been “tip and poured” when it was built on the inside seams. I own/run a few similiarly built tanks from that era of construction. Biggest worry/concern is that one seam. The “crazing” is all just yrs of water pressure and common to find for the age. You can tell by the color of the acrylic also and that extra thick/laminated top. They dont build em like that these days.
Since its already reinforced on the inside and theres some cracking in that reinforcement id put a patch on the outside of the tank running the height of the tank/seam. I prefer weld on #40 or #42. You will only need a pint and some scrap acrylic. Use a strip of material 2-3” wide covering the entire seam top to bottom and atleast 1/4” thick. This will ensure if the inside seal is comprimised the tank will still hold water with ur new patch/seal.
Yes, not filling the tank to the top will relieve some of the water pressure but i would most def. still do the outside patch on the problem seam and if any other seams look like that. For the cost of materials its well worth it. I run my older tanks around 4” from the top brim. Easy enough to redrill acrylic to lower the water level or cut an overflow box if needed.
