Hi Rui
for 1000lt I wouldnt bother with fibreglass...I would just use epoxy resin. Pour the resin onto an internal flat side of the aquarium...wait for it to go slightly tacky then rotate the aquarium and poor another internal flat surface.
I will pm you a link so you can have a read up on it.
If Im completely honest I would probably bite the bullet and go full glass for 1000ltrs @ 1000 euros. The extra 550 euros could easily account for your time and peace of mind.
Have a read up on the pour method and then you can decide.
Whilst the fibreglass looks smooth it certainly isnt smooth enough for a glass scraper. Yes I do get coraline algae, yes it does annoy me. I tried on a couple of occasions to don the wet suit and clean the back walls with a scrubbing pad but its difficult especially with corals in the tank so Ive resigned myself to putting up with the algae. In time I hope the coral growth will make it less noticeable.
The water level in the sump is controlled by a float switch that is located at water level when the surge tank is full and just about to fire. This is the lowest level of the sump running height, any lower would be due to evapouration. Therefore when the surge fires the float switch in the sump becomes submerged and is submerged until the surge tank is refilled. You need a reasonably quick ATO pump to replenish fresh ro water because theres only a small window for it to do so. Using my surge as an example it would fire every 7 minutes and would last 20-30 seconds. So the ATO float switch would be submerged for nearly 7 minutes and then would only have a few seconds to recognise any evaporation and to replenish the water. If the ATO pump was too slow it would struggle to keep up with small windows of opportunity.
I try and check my sg once/twice a week and never noticed any salinity issues with the ATO working in this fashion.
Im currently not running the surge because the flow was too much, it stirred up the tank entirely and made it cloudy. I have some large grain sand to distribute over the DSB to avoid the cloudy water but I just havent gotten around to sorting it out.
HTH