How do I prevent tanks in use from being removed?

I want to edit the script so that destroyed tanks in the game are removed but all the tanks that are being in used are left unaffected. Here's the current code.

local obj = Workspace['IS-4M']; 
local clone = obj:Clone()

while true do
    wait(120) -- wait 5 minutes
    if Workspace:findFirstChild("IS-4M") then 
        Workspace['IS-4M']:Destroy()
    end

    local cclone = clone:Clone()

    cclone.Parent = Workspace
    cclone:MakeJoints()
end