As long as you don't exceed the payload limit you should be ok on the existing lashing eyes.
Mine is strapped down using all 6 of the lashing eyes.
500kg /6= 83.6 kg
Doesn't work that way Marc, in a collision the lashing eyes to the front of the tank wont be taking any of the force
also if the tank isn't in a metal frame then it will buckle change shape meaning one strap could end up taking the full
force and snap overloading the other two.
With a bolted down tank in a frame the three bolts to the back will take the majority of the force but the three to the front
will still take a certain amount depending on the size of the plate under the floor, if one bolt snaps you will still have the opposite
bolt holding on unlike a strap where you lose the two.
Strapping a tank in is a quick and cheaper way to do the job and will allow a certain amount of protection but its definitely a
compromise on safety.