Got an MQ-510 PAR Meter. If you're interested in checking your tank's PAR, I'm happy to make a trip to check it. I'm in the city so nothing too far (max-1 hour). Fee would be $25-35 depending on distance. Let me know if interested.
You could get two of these side by side on a wood platform with a pump connecting the 2 to move water between the 2 tanks. And then you could connect a hose to the plumbing to do water changes. They’re 18” in diameter so would fit right in that space