New Plastic Tank

[cruzer]

I'm not a beginner but I am replacing a glass tank. 
I recently acquired a plexiglass 90 gal tank, 48x30x14. It was sitting in a covered garage and I don't know much of it's history. I currently have it outside full of water and it hasn't leaked yet. Should i reseal the seams? if so with what? I am going to fill it with RO water. If I use some of the water from my established tank will that reduce the time that I can put fish in it? The walls re made of 3/8 plexi and the back is 1/2". The front bulges a little from the weight but it seems normal. anything else I should do?

 

[StevenF]

 
 
Should i reseal the seams? 

There are no seams in a plastic tank .  The 5  panels of the tank are chemically welded together.  The glue used to melts the plastic and then the  parts are pressed together until the plastic hardens again.  If the tank is not leaking it probably never will.
 

 
 
The front bulges a little from the weight but it seems normal. anything else I should do?

this is more of a concern.  How much is it bowing out?  Normally There should be a plastic brace across the top to prevent the front and back panels from bowing  You might want to add one   IF the panel bows out too much it might crack.
 

 
 
going to fill it with RO water

I would recommend adding a fertilizer to the RO water before cycling it.   RO water has no macro or micro elements.  Bacteria need these elements to grow.  Without them it may take a long time to get the tank to cycle.

 

[Munroco]

If I use some of the water from my established tank will that reduce the time that I can put fish in it?

You can, but it will be a lot more effective if you can also swap over some of your filter material. That could give you an almost instant cycle.

 

[cruzer]

I am running the filter on my 50g tank. How long should I run it to get the benefits. The front doesn't bow out much much but looking for it you can see it. It does have a top welded on with holes for cleaning. I currently have a common pleco which when he grows too big I take them out to the koi pond, 5 angelfish about about 4 inches high, two kissing gouramis, one pygmy frog. what would be something different I can put in with these fish. 
Thank You for your help.