Hello; The new pictures help. Looks like you could clad the legs on the outside corrners with additional 2x4 lengths. The outside of each leg will need two lengths of 2x4. A short one from the floor up to the first horozontal brace and a longer one from that brace up to the top horozontal brace. If cut correctly for a tight fit, then the load should transfer from the tank down to the floor along the wood. This could be done on each outside corrner. This could possibly be done as the stand sits for all but the back side near the wall. At least the stand and tank might survive as it sits, but there could be a risk.
To make the cuts both square and at a fairly exact length will be difficult with freehand tools. It can be done but expect some wasted lumber. With a table saw it should be somewhat easier to make true cuts and repeat them once the fence is set to the correct length.